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().

Commit 3a212412 changed the meaning of out_audio_format, but I forgot
to apply this to ConvertFilter::FilterPCM().

See https://bugs.musicpd.org/view.php?id=4551

Previously, volume was applied twice: once by PcmVolume, and again by
the hardware mixer.

For easier state management inside filter plugins.