Add gossip_queries methods to RoutingMessageHandler trait

Add gossip_queries methods to RoutingMessageHandler trait

Defines message handlers for gossip_queries messages in the RoutingMessageHandler
trait. The MessageSendEventsProvider supertrait is added to RoutingMessageHandler
so that the implementor can use SendMessageEvents to send messages to a
peer at the appropriate time.

The trait methods are stubbed in NetGraphMsgHandler which implements
RoutingMessageHandler and return a "not implemented" error.