Let the purge_coordinator_task acquire purge_sys.rseg->latch
less frequently and hold it longer at a time. This may throttle
concurrent DML and prevent purge lag a little.

Remove an unnecessary std::this_thread::yield(), because the
trx_purge_attach_undo_recs() is supposed to terminate the scan
when running out of undo log records. Ultimately, this will
result in purge_coordinator_state::do_purge() and
purge_coordinator_callback() returning control to the thread pool.

Reviewed by: Vladislav Lesin and Vladislav Vaintroub