Make net_write_timeout work with backend connections as well

The net_write_timeout parameter controls how long a session may have buffered writes on the client TCP socket. This does not help avoid hanging clients if the TCP connection to the backend is blocked. Having the parameter control both client and backend TCP connections would make the behavior consistent and it would prevent clients from hanging if the network between MaxScale and the backend database server is blocked for client traffic but the connection that the monitor uses is not blocked or if no monitor is used at all.