MySQL GET_LOCK behaves differently in MySQL < 5.7.5 and MySQL >= 5.7.5

[Josh Di Fabio]

We probably need different implementations for MySQL < 5.7.5 and MySQL >= 5.7.5 since the behaviour of GET_LOCK changed significantly in 5.7.5 [1].

Furthermore, we should consider what happens if multiple locks are acquired in the same process as, prior to MySQL 5.7.5, a single connection could only hold a single lock [1].

[1] http://dev.mysql.com/doc/refman/5.7/en/miscellaneous-functions.html#function_get-lock