New INI setting live_query_max_execution_time to allow configuration of max execution time #14858

cc @mattab

Users can now optionally configure a max_execution_time for live queries to prevent a live query from running for a very long time eg when looking at a year with a segment that doesn't find any visitors.

Added as many tests as I could. Noticed the MySQL version on Travis doesn't support this feature yet otherwise could have tested it there further.

Unfortunately this syntax isn't supported in MariaDB which has another synax to set the maximum execution time for a query:
https://mariadb.com/kb/en/library/aborting-statements/

thanks @Findus23 I will mention in the config that it likely won't work on MariaDB

Added some tests @diosmosis