gbproxy: Add NSE peer that can have multiple gbproxy_peers

We want this level of indirection to support multiple BVCs per NSE. The
current code assumes that an NSE only has one BVC which breaks messages
on the signalling BVC which should only be sent once to an NSE
regardless of the number of BVCs it contains.

Change-Id: I97cc6c8f8c0f1b91577ab8f679c4ae217cc88076
Related: SYS#5226
9 files changed