server: add message delivery between peers

a peer notifies other peers of its event (join,leave,bgp state change,
updates, etc)

It would have better performance for a peer to send a message to a
channel of another peer directly. However, for the code simplicity,
the hub-and-spoke model is used for now.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>