[4.0.0-alpha17] Fix lock! thread safety performance

Fixes

• https://github.com/rainforestapp/queue_classic_plus/releases/tag/v4.0.0.alpha14 caused a performance issue with too much locking on the queue. This is re-implements thread safety on lock! without using transaction locks.

Changes

• QUEUE_CLASSIC_MAX_LOCK_TIME is no longer supported and jobs with lock! will not have a timeout. This feature was inherently dangerous in that a lock! job should never have more than 1 instance enqueued.