MBS-7197: clear release cache on release group merges. Also, cached types on artist credit replacements.
Merge branch 'mbs-7197'

* mbs-7197:
  MBS-7197: clear release cache on release group merges. Also, cached types on artist credit replacements.
Merge pull request #2 from metabrainz/master

Merge updates from metabrainz:master