msgpack rpc helper

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
rpc: copyright notice
rpc: fix msgid overflow
rpc: comments and assertions
rpc: make create_request return msgid as well
rpc: client class
rpc: some more
rpc: long line
rpc: make recv smarter
rpc: move some msgpack specific bits to Encoder
rpc: simplify
rpc: reorder code a bit
rpc: comment
rpc: coalesce get_messages and dispatch_messages
rpc: simplify
rpc: minor tweak
rpc: ignore messages with unknown type
rpc: default params for response.  add send_notification method.
rpc: comment, whitespace
rpc: minor tweak