Update MPD v0.20.x series to 0.21.5

Update from 0.21.4 to ver 0.21.5 (2019/02/22)

• protocol
  • fix deadlock in "albumart" command
  • fix "tagtypes disable" command
• database
  • simple: fix assertion failure
  • fix assertion failures with mount points
• storage
  • udisks: fix "AlreadyMounted" error
  • udisks: use relative path from mount URI
  • fix memory leak
• input
  • buffer: fix crash bug when playing remote WAV file
• tags
  • ape: map "Album Artist"
• output
  • shout: add support for TLS
• mixer
  • pulse: add "scale_volume" setting