Race condition in the index optimization logic

The index optimization in JIRA is scheduled after a given (configurable, 4000 by default) number of updates to a given index. The logic behind introduces a race condition in that multiple optimizations may be scheduled after the threshold was exceeded once. This may happen if there is more than one simultaneous request that results in update of given index (e.g. multiple create/edit issue requests for issue index). This generally slows down index updates and affects overall throughput of JIRA. Together with JRA-21578 this may potentially lead to serious performance degradation under some very specific circumstances.