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.

LGTM :+1: (Benaka left a comment above)

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