Reduce overhead by eliminating MultipleOutputs::ResetReopen().

.. and remove the obsolete method OpenFilter().

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

Try harder to skip steps (reopen filter, reopen output) if the
AudioOutput is already open.

Un-protect the SharedPipeConsumer instance, and make it available in
the OutputThread only.  This gives more well-defined mutex protection.

Make in_audio_format accessible only from within the OutputThread, and
add a new one for inter-thread communication.

Reduce latency by stopping all AudioOutputs asynchronously.

Use ScopeLock to manage the lock; don't unlock after obtaining the
"really_enabled" flag; keep the same lock during EnableWait() /
DisableWait().

Simplify the code, eliminate duplicate rollback code and eliminate the
"goto" kludge.

Remove dependency on ReplayGain global variables.

With the configuration "replay_gain_handler none", the Filter pointers
are never initialized, which can crash MPD.

 https://bugs.musicpd.org/view.php?id=4571

For easier state management inside filter plugins.