Allows defining a list of supported audio formats, and allows
switching on and off DoP with certain formats.

This is a first rough draft.  The setting syntax and its semantics may
still be redesigned.

Needed for WStringView.

We can do CURL requests asynchronously, and we don't need a
synchronous WorkQueue thread for that.

This allows parallelizing lookups and allows immediate cancellation.

Our local EventLoop instance was never run; it was not usable.

Yet another C-style vtable replaced with C++.

Hide struct FilteredAudioOutput from the plugins, preparing for hiding
MPD's core internals.

We will reuse these tags elsewhere.

The first step to multi-player support.  Not much, just a dummy
command.

Eliminate global variables.

Use SND_PCM_NONBLOCK, and perform all snd_pcm_writei() calls in the
IOThread.  Use a lockless queue to copy data from the OutputThread to
the IOThread.

This rather major change aims to improve MPD's internal latency.  All
waits are now under MPD's control, instead of blocking inside
libasound2.

As a side effect, an output's filter is now decoupled from the actual
device I/O, which solves a major latency problem with the conversion
filter on slow CPUs and small period buffers.  See:

 https://bugs.musicpd.org/view.php?id=3900