LPS-47303 (Fix tests) Comments API: transfer all Message Boards references from BaseIndexer

Hi @brianchandotcom,

Fixed https://github.com/brianchandotcom/liferay-portal/pull/19105#issuecomment-45566553 so tests pass again. Last commit changes BaseIndexer implementation very slightly to make it more evident that order matters. If you prefer it as it was before, just ignore the last commit. If you'd like me to squash the whole PR in a single commit, close with a comment and I'll resend.

Thanks!