addr protocol && start of sync protocol (#25)

* p2p <-> sync interfaces proposal

* updated with example

* send errors will be handled in p2p module => no need to return to the sync

* poc of outbound sync connection

* simplified send_to_peer

* context has cpu pool and enent loop handles

* on_message won't return ProtocolAction anymore

* session initialized sync protocol, remove retain cycles on P2P::drop

* removed ProtocolAction

* uncommented ping protocol

* node_table sorts nodes also by recently used time

* send getaddr on connect

* fixed node_table insert, added insert_many

* addr protocol

* added TODO: remove