bgpd: Use synchronous way to get labels from Zebra

Both the label manager and table manager zapi code send data requests via zapi
to zebra and then immediately listen for a response from zebra. The problem here
is of course that the listen part is throwing away any zapi command that is not
the one it is looking for.

ISIS/OSPF and PIM all have synchronous abilities via zapi, which they all
do through a special zapi connection to zebra. BGP needs to follow this model
as well. Additionally the new zclient_sync connection that should be created,
a once a second timer should wake up and read any data on the socket to
prevent problems too much data accumulating in the socket.

```
r3# sh bgp labelpool summary
Labelpool Summary
-----------------
Ledger:       3
InUse:        3
Requests:     0
LabelChunks:  1
Pending:      128
Reconnects:   1
r3# sh bgp labelpool inuse
Prefix                Label
---------------------------
10.0.0.1/32           16
192.168.31.0/24       17
192.168.32.0/24       18
r3#
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>

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