Thanks to Sergey Vojtovich for feedback and many ideas.

purge_state_t: Remove. The states are replaced with
purge_sys_t::enabled() and purge_sys_t::paused() as follows:
PURGE_STATE_INIT, PURGE_STATE_EXIT, PURGE_STATE_DISABLED: !enabled().
PURGE_STATE_RUN, PURGE_STATE_STOP: paused() distinguishes these.

purge_sys_t::m_paused: Renamed from purge_sys_t::n_stop.
Protected by atomic memory access only, not purge_sys_t::latch.

purge_sys_t::m_enabled: An atomically updated Boolean that
replaces purge_sys_t::state.

purge_sys_t::running: Remove, because it duplicates
srv_sys.n_threads_active[SRV_PURGE].

purge_sys_t::running(): Accessor for srv_sys.n_threads_active[SRV_PURGE].

purge_sys_t::stop(): Renamed from trx_purge_stop().

purge_sys_t::resume(): Renamed from trx_purge_run().
Do not acquire latch; solely rely on atomics.

purge_sys_t::is_initialised(), purge_sys_t::m_initialised: Remove.

purge_sys_t::create(), purge_sys_t::close(): Instead of invoking
is_initialised(), check whether event is NULL.

purge_sys_t::event: Move before latch, so that fields that are
protected by latch can reside on the same cache line with latch.

srv_start_wait_for_purge_to_start(): Merge to the only caller srv_start().