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.
fixes #158 Use promotion to record the blocknum of the rightmost
non-root leaf node in each FT. When the FT detects a rightmost insertion
pattern, it attempts to do inserts and unique checks directly into the
rightmost leaf node, greatly optimizing sequential insert speed.