lib/rpc.py: break serve loop if socket closed by peer

bug: serve中にコネクションが切れるとno-waitのループになる.
fix: コネクションが切れたらループを抜けてソケットを閉じる.
base app of RPC manager

- RPC server accept
- manage server endpoint apps
base app of RPC endpoint

- echoing to peer socket
- socket closed by peer then send_observer EventRPCEndpointClosed
services/vrrp: RPC sample code

- services/vrrp/rpc_manager.py: RPC manager
- services/vrrp/rpc_endpoint.py: RPC endpoint