[FEATURE] Generate warning when dangerous settings are changed and/or detected

[renecannao]

This is a placeholder. Further tickets will be created.

We need a mechanism to report dangerous setting when they are changed or detected.

For example we could generate warnings in the error log, and also track when these changes were applied.

Some example may include:

• mysql-auto_increment_delay_multiplex (when 0)
• mysql-query_digests (when disabled)
• mysql-wait_timeout (when configured below a certain threshold)
• mysql-use_tcp_keepalive

We could also generate warning for not recommended (but safe) changed, for example:

• mysql-query_digests_track_hostname : warning that it could cause significant memory usage
• mysql-query_digests_keep_comment : warning that it could cause significant memory usage
• mysql-stats_time_backend_query : warning about CPU overhead
• mysql-stats_time_query_processor : warning about CPU overhead
• mysql-hostgroup_manager_verbose : warning about reduced visibility

We can also highlight the danger of changing these variables in the documentation pages.

[noizu]

@renecannao this was scheduled out in April, were sub issues related to this opened? Anything that would be going in v3.0.3

[noizu]

Moving to 3.0.4 but can be punted to 3.0.5