Apply backpressure when we have too many gossip checks in-flight

Apply backpressure when we have too many gossip checks in-flight

Now that the `RoutingMessageHandler` can signal that it needs to
apply message backpressure, we implement it here in the
`PeerManager`. There's not much complicated here, aside from noting
that we need to add the ability to call `send_data` with no data
to indicate that reading should resume (and track when we may need
to make such calls when updating the routing-backpressure state).