Journal lmdb joint

Proposal of LMDB-based journal.

Key features:

• all journals for all zones are in a single LMDB database (defaults to storage/journal; 1G size)
• the occupied space is measured per zone
• old changesets get preserved after zone flush until we run out of space
• if zone flushing is disabled and journal gets full, it tries to free up space by merging older changesets
• all changes are done by transactions, resulting in always-consistent DB (but some mutexes still necessary for opening DB && for keeping zone contents consistent with journal)
• kjournalprint provides a way to list zones in journal
• old journal is automatically imported, but the configuration needs to be updated manually

Caveats:

• not tested enough, needs experience from some "real" env