Refactor server status delivery to routers

When the server status changes, routers would sometimes need to react to if configured in certain ways. For example, in readwritesplit the loss of the Write status implies that the server is no longer valid for writes and connections to it should be closed. Similarly, connections to servers in maintenance mode should be closed.

This is both a performance improvement as well as a correctness one which removes the need for the constant checking of maintenance status.

MXS-6864 added a hangup that's delivered under certain conditions to prevent a transaction from committing on a node that had lost the master lock. With this mechanism, the router itself could do the hangup.