purge_sys_t::n_submitted: Document that it is only accessed by
srv_purge_coordinator_thread.

purge_sys_t::n_completed: Exclusively use my_atomic access.

srv_task_execute(): Simplify the code.

srv_purge_coordinator_thread(): Test the cheaper condition first.

trx_purge(): Atomically access purge_sys.n_completed.
Remove some code duplication.

trx_purge_wait_for_workers_to_complete(): Atomically access
purge_sys.n_completed. Remove an unnecessary local variable.

trx_purge_stop(): Remove a redundant assignment.