zebra: Bring up 514 BGP neighbor sessions

zebra: Bring up 514 BGP neighbor sessions

Issue:
When 514 inerfaces/neighbors are configured, it creates socket error,
"Cannot allocate memory", when back to back V6 RA messages are tried
to be sent over the socket. This prevents interface, to know its peer's
link local address. Socket error comes when 1) try to join ICMPv6 all
router multicast group, back to back for all interfaces 2)send back to
back RA for all interfaces

Fix:
1)For ICMPv6 join case, we check if the interface has already joined
all router group, if not try to join. On failure, retry joining after
random amount of time determined 1 ms to ICMPV6_JOIN_TIMER_EXP_MS(100 ms)
2) For RA issue case, batch sending of RA mesages using wheel timer

Testing:
Monitor BGP session running sh bgp summary command

Before fix:
r1# sh bgp summary

IPv4 Unicast Summary:
BGP router identifier 192.168.1.1, local AS number 1001 VRF default vrf-id 0
BGP table version 0
RIB entries 0, using 0 bytes of memory
Peers 515, using 12 MiB of memory

Neighbor        V         AS   MsgRcvd   MsgSent   TblVer  InQ OutQ  Up/Down State/PfxRcd   PfxSnt Desc
r1-eth0         4       1002        89        90        0    0    0 00:07:10            0        0 N/A
r1-eth1         4       1002        89        90        0    0    0 00:07:10            0        0 N/A
r1-eth2         4       1002        89        90        0    0    0 00:07:10            0        0 N/A
r1-eth3         4       1002        89        90        0    0    0 00:07:10            0        0 N/A
r1-eth4         4       1002        89        90        0    0    0 00:07:10            0        0 N/A
r1-eth5         4       1002        89        90        0    0    0 00:07:10            0        0 N/A

…..<snip>...
r1-eth252       4       1002        31        29        0    0    0 00:02:08            0        0 N/A
r1-eth253       4       1002        31        29        0    0    0 00:02:08            0        0 N/A
r1-eth254       4       1002        31        29        0    0    0 00:02:08            0        0 N/A
r1-eth255       4       1002        31        29        0    0    0 00:02:08            0        0 N/A
r1-eth256       4          0         0         0        0    0    0    never         Idle        0 N/A
r1-eth257       4          0         0         0        0    0    0    never         Idle        0 N/A
r1-eth258       4          0         0         0        0    0    0    never         Idle        0 N/A
r1-eth259       4          0         0         0        0    0    0    never         Idle        0 N/A
r1-eth260       4          0         0         0        0    0    0    never         Idle        0 N/A
……..<snip>…..
r1-eth511       4          0         0         0        0    0    0    never         Idle        0 N/A
r1-eth512       4          0         0         0        0    0    0    never         Idle        0 N/A
r1-eth513       4          0         0         0        0    0    0    never         Idle        0 N/A
r1-eth514       4          0         0         0        0    0    0    never         Idle        0 N/A
After fix:
r1# show bgp summary

IPv4 Unicast Summary:
BGP router identifier 192.168.1.1, local AS number 1001 VRF default vrf-id 0
BGP table version 0
RIB entries 0, using 0 bytes of memory
Peers 515, using 12 MiB of memory

Neighbor        V         AS   MsgRcvd   MsgSent   TblVer  InQ OutQ  Up/Down State/PfxRcd   PfxSnt Desc
r1-eth0         4       1002        87        87        0    0    0 00:07:04            0        0 N/A
r1-eth1         4       1002        87        87        0    0    0 00:07:04            0        0 N/A
r1-eth2         4       1002        87        87        0    0    0 00:07:04            0        0 N/A
r1-eth3         4       1002        64        67        0    0    0 00:05:09            0        0 N/A
r1-eth4         4       1002        87        87        0    0    0 00:07:04            0        0 N/A
r1-eth5         4       1002        87        87        0    0    0 00:07:04            0        0 N/A
r1-eth6         4       1002        67        70        0    0    0 00:05:22            0        0 N/A
r1-eth7         4       1002        87        87        0    0    0 00:07:04            0        0 N/A
r1-eth8         4       1002        87        87        0    0    0 00:07:04            0        0 N/A
....
r1-eth499       4       1002        43        43        0    0    0 00:03:22            0        0 N/A
r1-eth500       4       1002        43        43        0    0    0 00:03:22            0        0 N/A
r1-eth501       4       1002        19        22        0    0    0 00:01:21            0        0 N/A
r1-eth502       4       1002        43        43        0    0    0 00:03:22            0        0 N/A
r1-eth503       4       1002        43        43        0    0    0 00:03:22            0        0 N/A
r1-eth504       4       1002        20        23        0    0    0 00:01:30            0        0 N/A
r1-eth505       4       1002        43        43        0    0    0 00:03:22            0        0 N/A
r1-eth506       4       1002        43        43        0    0    0 00:03:22            0        0 N/A
r1-eth507       4       1002        22        25        0    0    0 00:01:39            0        0 N/A
r1-eth508       4       1002        43        43        0    0    0 00:03:22            0        0 N/A
r1-eth509       4       1002        17        20        0    0    0 00:01:13            0        0 N/A
r1-eth510       4       1002        43        43        0    0    0 00:03:22            0        0 N/A
r1-eth511       4       1002        43        43        0    0    0 00:03:22            0        0 N/A
r1-eth512       4       1002        19        22        0    0    0 00:01:22            0        0 N/A
r1-eth513       4       1002        43        43        0    0    0 00:03:22            0        0 N/A
r1-eth514       4       1002        43        43        0    0    0 00:03:22            0        0 N/A

Signed-off-by: Soumya Roy <souroy@nvidia.com>