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

The filter can take some time to finish, even more so on a weak
machine with a saturated CPU.  By not holding the mutex during that
time, we can reduce PlayerThread latency a lot, because that thread
needs to synchronize a lot with all outputs.

Don't expose MusicChunk instances, provide higher-level access to
chunk contents.