React to redirect_url from backends

If the MariaDB server sends a packet with a redirect_url in it, MaxScale should react to it somehow.

Currently MaxScale ignores it and does nothing. This causes the client to potentially redirect away from MaxScale. This doesn't seem like behavior that's really desirable if the clients are supposed to connect via MaxScale.

Removing the redirection from the packet and putting the server into either Maintenance or Draining states would be better. This has the benefit of doing what's expected of a "client" when the server is redirecting connections. The removal could be implemented as either a removal of the variable from the packet or by mangling the name into something with the same length (e.g. from redirect_url to org_redr_url) to avoid having to modify the packet length. The latter option would also have the benefit of still showing the information to the client.