Fixes a potential deadlock introduced by commit 94528735

Using variable-size objects is not worth the trouble here.  Let's drop
this and use existing and simpler code.

Fixes another buffer overflow: if the stream has a very long title or
URL, resulting in a metadata string of more than 2 kB, icy_string[0]
is a negative value, which gets casted to size_t - ouch!

 https://bugs.musicpd.org/view.php?id=4652

Fixes a buffer overflow due to the bad formula rounding the buffer
size up.  At the same time, remove the "+1" from the meta_length
calculation, which takes the padding into account and at the same time
implements proper rounding.

Why did this cast exist??

During UnlockActivate() while the mutex is unlocked, the IOThread can
set a new error condition, and will never again wake up the
OutputThread.  This race condition can cause a deadlock in the
OutputThread.

.. and not the input data.

Regression from commit b1c7649e (integer underflow).

 https://bugs.musicpd.org/view.php?id=4639

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

Same as in commit e02d8ad8, but this time for the sndio plugin
which can be emulated by libroar.

Eliminate dependency on io_thread_get().

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

If the input AudioFormat changes but the out_audio_format doesn't
change (e.g. because there is a fixed "format" setting in this
"audio_output" section), the ConvertFilter needs to be reconfigured.
This didn't happen, resulting in awful static noise after changing
songs.