The API to allow storage change introduced a mutex in the OperationContext destroy path that increased median latency but decreased 95% latency in a perfomance microbenchmark (standalone inMemory genny_scale InsertRemoveTest.Remove). This replaces the mutex acquire with a simple test on OperationContext destruction; the mutex is now only acquired during storage change.

This also closes a narrow race where the observer could be called before the storage change, the OperationContexts iterated, and then an OperationContext released without going through the StorageChangeContext::onOperationContextDestroy code. This would result in a hang.