pim-kat: changed kat handling to match rfc-4601 more closely.

pim-kat: changed kat handling to match rfc-4601 more closely.

1. This is needed to layout the MSDP macros for determining what SAs are
originated by a MSDP speaker.
2. We no longer let the kat timer expire on an active flow. Activity
counters/lastuse is polled via a wheel for every SG entry. If new
activity is detected the keepalive timer is started and SPT bit set.
A SRC_STREAM reference is also created for the entry if one doesn't
already exist.
3. If KAT actually expires it means the flow is no longer active. At
this point we stop advertising the SA to MSDP peers. We also pull
the SRC_STREAM reference (deleting the entry if there are no other
references).

PS: Checking counters on KAT expiry will come in the next change.

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>
Acked-by: Donald Sharp <sharpd@cumulusnetworks.com>