Remove dependency on ReplayGain global variables.

 https://bugs.musicpd.org/view.php?id=4595

Don't use the global variables in class DecoderBridge; instead,
forward these values to the decoder thread via PlayerControl and
DecoderControl.

Move code from ReplayGainConfig.cxx.

Declare two named elements.  An enum should not be used as an array
index, as this is error prone.

Eliminates Complete().

Move code from replay_gain_get_real_mode().

SimpleDatabase::Visit() always throws exception when visiting a song.
Adding a song to the queue always results in a "No such directory"
error response, although the song is successfully added.  This
behaviour was introduced by commit
3ff728ab.  This patch fixes this.

https://bugs.musicpd.org/view.php?id=4601

Speed up container_scan() again, by eliminating the need to call
scan_file() for each item.

Broken by commit 0abee77e

Got lost in 0abee77e