Log Message:
musicpd: update to 0.21.5.

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