master, metarestore: Add changelogs migration to new format

When upgrading from version before 1.6.29 all changelog files
changelog.X.mfs will be renamed to changelog.mfs.X during master server
start, and changelog_ml.X.mfs will be renamed to changelog_ml.mfs.X
during metalogger server start.

Change-Id: If4f9181a6e6524ebe1f114cb0cc366496d5a68db