purge_sys_t::m_enabled transition to std::atomic.

enabled_latched() doesn't make much sense: in this particular case it is
as fast as atomic load. The sole caller has to reload it's value anyway,
due to rw_lock_x_lock(&purge_sys.latch) issuing acquire memory barrier.

When purge_sys_t::close() is reached, m_enabled must be false, otherwise
we may free members, which are still in use by the coordinator thread.