Add send message events for gossip_queries

This change enables initiating gossip queries with a peer using the
SendMessageEvent enum. Specifically we add an event for sending
query_channel_range to discover the existance of channels and an event
for sending query_short_channel_ids to request routing gossip messages
for a set of channels. These events are handled inside the process_events
method of PeerManager which sends the serialized message to the peer.