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.
Merge pull request #180 from Tokutek/rightmost_leaf

Optimize rightmost inserts and unique checks using a constant rightmost leaf and a heuristic stored in the FT