purge_coordinator_timer_callback(): Remove. We will have
purge_coordinator_timer invoke purge_coordinator_callback()
directly.

srv_master_callback(): Invoke srv_wake_purge_thread_if_not_active()
instead of purge_coordinator_timer_callback(). That is, we will
trigger purge_coordinator_callback() once per second if there is
any work to be done.

purge_state::do_purge(): Replaces srv_do_purge(),
purge_coordinator_callback_low(), and
purge_coordinator_timer_callback(). The static variables
inside srv_do_purge() were moved to purge_state data members.