fixes #248 Convert to a tree on omt clone if it must support marks
fixes #226 When serializing a nonleaf node, include the offsets stored
in each message tree. This removes a sort during deserialization, which
can be expensive when there are many messages and I/O is fast. This
change supports auto-upgrade from older versions.
changed CHECKPOINT_DURATION[_LAST] to UINT64 #249

fixes #249