In the PoC of parallel checkpoint a new mechanism was added to perform relaxed loads of the queue head to see if had any items prior to locking the queue. This is an optimization and while it seems obvious I'd like to check if it actually nets us any perf gains prior to keeping it permanently. 

To do this we need to remove all locations of it, run a multipatch build and see if the perf with / vs without is different. If we lose perf then we should keep it.

In the even we keep it we should rename the macro from WT_TAILQ_EMPTY_TSAN_SUPPRESS to WT_TAILQ_EMPTY_UNSAFE, or WT_TAILQ_EMPTY_RELAXED.

Scope:

• Investigate perf benefit and remove or rename.