Clean up _revision entries on commit root documents

The _revisions entries on commit root documents are currently not cleaned up and accumulate in split documents.

One possible solution may be to ensure that there are no uncommitted changes up to certain revisions. Older revisions would then be considered valid and commit information on the commit root document wouldn't be needed anymore.

For regular commits this is probably not that difficult. However, changes from branch commits require the merge revision set in the commit entry on the commit root to decide when those changes were made visible to other sessions. A simple solution could be to rewrite such changes with the merge revision.