ReadWriteSplit should write warning to log if causal_reads is set when requirements not met

The causal_reads parameter of ReadWriteSplit has the following requirements:

Note: This feature requires MariaDB 10.2.16 or newer to function. In addition to this, the session_track_system_variables parameter must be set to last_gtid.

https://mariadb.com/kb/en/mariadb-maxscale-23-readwritesplit/#causal_reads

It might be helpful for troubleshooting if ReadWriteSplit could write a warning to the error log if causal_reads is enabled when these requirements are not met.