Continue synchronization protocol impl (#30)

* 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

* started work on sync package

* send getaddr on connect

* fixed node_table insert, added insert_many

* addr protocol

* sync headers response

* continue sync

* continue sync protocol

* separated sync connection start from sync connection creation

* switched to getblocks (instead of getheaders)

* starting to receive blocks in sync

* deal with orphaned blocks in sync::local_chain

* continue sync

* fix warnings

* removed TODOs

* protocol::sync is back to private state

* fixed grumbes