Eliminate dependency on io_thread_get().

Eliminate dependency on io_thread_get().

With "ok==false", newly created threads may quit instantly.

libupnp provides seconds, not whatever time unit is used by
std::chrono::steady_clock.

If the base class is not accessible, the "catching" the base class
won't work.  This caused the fatal error:

 terminate called after throwing an instance of 'LibmpdclientError'

Each close/open cycle resets the Filter's state, because a new Filter
instance is being created.  That results in the serials
(replay_gain_serial and other_replay_gain_serial) being out of sync
with the internal ReplayGainFilter state.

So instead of initializing those serials once, we need to initialize
them each time we create new ReplayGainFilter instances, i.e. in
OpenFilter().

 https://bugs.musicpd.org/view.php?id=4632