Reduce overhead by eliminating MultipleOutputs::ResetReopen().

Eliminate a dependency from MultipleOutputs on PlayerControl.

Reduce latency by stopping all AudioOutputs asynchronously.

Reduce shutdown latency by two commands per output.

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

Remove dependency on ReplayGain global variables.

The old struct config_param remains only for top-level string options.

-fixes SIGABRT when mpd is started from a previously paused state
-regression introduced by commit: f5a923b9:
'OutputAll: convert to class, move instance to class Partition'

-input_audio_format was previously declared using the static modifier
ensuring it was zero-initialised by default
-the current default-initialised input_audio_format contains garbage at
runtime which allows the AudioFormat.IsDefined() 'fail fast' test in
MultipleOutputs::Update to pass and the SIGABRT follows in
AudioOutput::Open when passed the invalid input_audio_format struct
-switching AudioFormat.IsDefined() for AudioFormat.IsValid() is an
alternative workaround

Use a listener interface instead of GlobalEvents.

Another big chunk of code for multi-player support.