Provide an option to enable/disable new get_lock/release_lock implementation introduced in 5.6.19

[Renjish Abraham]

With 5.6.19, get_lock/release_lock was re-implemented with MDL with a significant departure from existing behavior of get_lock function. Prior to 5.6.19, get_lock always gets only one lock for the named lock and closes any previously opened lock held by the same connection. The new implementation makes get_lock calls for the same connection if already acquired to be more like a counter and also allows multiple different named locks to be created via get_lock by the same connection. This is a significant departure from documented behavior and IMHO should be a feature for a major release rather than minor release. Based on old logic prior to 5.6.19, there are many client applications including ours where we attempt to probe absence or presence of lock via get_lock/release_lock calls and this can possibly cause unexpected outcome.

Providing an option to enable/disable the new implementation with old implementation being the default will be a good feature to have.

[Valerii Kravchuk (valerii-kravchuk)]

This is a duplicate of bug #1401528 that I've reported based on your support issue on December 11, 2014.