Read with max_slave_connections=0 fails with master_failure_mode=fail_on_write

If the current master fails and readwritesplit is configued in the following way, the read will fail.

[ReadWriteSplit]

type=service

router=readwritesplit

max_slave_connections=0

master_reconnection=true

master_failure_mode=fail_on_write

This problem is related to MXS-5085 where a read after a switchover did not correctly select the new master. When MXS-5085 is fixed, this problem could be solved by temporarily allowing a slave connection to be created if a valid root master is not found and the router is configured with max_slave_connections=0. However, this will cause an extra slave connection to remain in use even once a new master connection is created.