zebra: Fix label manager proxy mode.

zebra: Fix label manager proxy mode.

The current implementation did not consider multiple clients to
a label-manager acting as proxy, i.e. relaying messages to another
label manager. Specifically, upon a client's request, it checked
the socket & buffer from the actual label manager for pending
responses and directly copìed them to the client --currently--
being served. As a result, if two clients (e.g. ldpd and bgpd)
sent requests, it could happen that responses being 'on the wire'
from the real label manager towards the proxy, where relayed to
the wrong client. This patch, which requires all msgs to include
a a proto & instance pair, lookups up the zserv client that a
message (response) is to be relayed to.

Signed-off-by: Fredi Raspall <fredi@voltanet.io>