Add a more check for thread_handling=pool-of-threads

[svaroqui]

add an option

reject-write=max_connection | read_only default lower connection to 1

if max_connection is set then thread_handling=pool-of-threads to pass failover check extra-port= is check to be the same as mrm connection port

Maxscale task https://jira.mariadb.org/browse/MXS-778

in read_only mode if MXS-778 is implemented Would need master freeze in monitor.go to be waiting for at least 2 time the default monitoring time of maxscale before entering the kill long running transaction loop