release v0.20.19

Closes #258

Fixes "search already in progress" errors.

See code comment.  Closes #260

Don't reactivate the PCM device immediately after Cancel() is
finished; if Cancel() gets called this may mean that new data may take
a while to produce, or no data at all will be produced because the
current song is being stopped.

Once new data is available, Play() will automatically reactivate the
PCM.

This fixes underruns when switching songs manually (closes #264).

Use the "active" flag in PrepareSockets() instead of LockHasError().

Prepare to make the "active" attribute thread-safe.

av_register_all() was deprecated in
FFmpeg/FFmpeg@0694d8702421e7aff1340038559c438b61bb30dd

When switching to another song manually, the player checks if the
decoder is already decoding that song; if so, it will attempt to reuse
it by seeking it to the new position.  That however fails if the
decoder is not seekable (e.g. a radio stream) which leaves the user
unable to switch to that song with the bogus error message "Not
seekable".

Closes #256

There was a discrepancy between what was written to the buffer and the
size returned by pcm_dsd_to_dop(): the "for" loop uses num_frames/2,
rounding down, while the return value is num_samples which is
num_frames*channels, without rounding.  This could cause undefined
data at the end of the destination buffer if the source buffer size
was not aligned to multiples of 8 bytes (4 DSD bytes per channel).

The latter however can occur in the 0.21 branch after commit
a06bf388

Closes #233

Fixes crash due to "pure virtual method called" in the "mms" input
plugin.  Closes #253

Closes #247

Without setting the "ready" flag, the caller will wait in WaitReady()
forever, locking up MPD.  Closes #252

lockfree library used by ALSA output plugin is part of Boost from version 1.53,
so this can be theoretically the lowest required version, however
there are issues which are resolved from 1.54 onwards.

Related to #196

First commit for issue #69