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