p2p: rework protocol API

p2p/connection.go

p2p/connection_test.go

p2p/message.go

p2p/message_test.go

p2p/messenger.go

p2p/messenger_test.go

p2p/peer.go

p2p/peer_error.go

p2p/peer_error_handler.go

p2p/peer_error_handler_test.go

p2p/peer_test.go

p2p/protocol.go

p2p/server.go

p2p/server_test.go