Remove should_request_full_sync from RoutingMessageHandler

Remove should_request_full_sync from RoutingMessageHandler

This method was used to set the initial_routing_sync flag when sending
an outbound Init message to a peer. Since we are now relying on
gossip_queries instead of initial_routing_sync, synchronization can be
fully encapsulate into RoutingMessageHandler via sync_routing_table.

This commit removes should_request_full_sync from the trait
RoutingMessageHandler. The implementation is still used in
NetGraphMsgHandler and has been converted into a private method instead
of a trait function.