Wired to Filter::Flush().

Closes #153

This will be used by filters which have internal buffers which need to
be flushed at the end, e.g. the "soxr" resampler.

Wohooooo, the method Filter::Reset() has been broken because no
implementation of it has ever been called for a loooong time.
And nobody ever noticed it.  WTF.

Thsi filter doesn't make sense as manually configured plugin, because
nobody ever calls volume_filter_set() on it, making it a no-op.

These are not in filter_plugins[] and have dedicated constructors
anyway.

Circumvent filter_new() with a fake ConfigBlock.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Not used.  Force implementations to initialize out_audio_format.

Remove dependency on ReplayGain global variables.

Eliminates Complete().