handle nullptr more correctly in bn_data
fix overactive inequality assertion in mempool
reimplemented bn_data::move_leafentries_to for speed

now uses dmt iterate_on_range and the dmt builder instead of a for loop
Comments, minor renames, reworked bn_data::move_leafentries s.t. splits do not significantly increase memory usage of basement nodes and to be cleaner
cleaned up serialization, upped version to 26
improve test coverage, minor formatting
don't (malloc|alloca)+memcpy before le_pack; Instead just keep mempool around a little longer (like pre-refactoring).  This should
reduce large mallocs and remove a memcpy
Unify toku_mempool_*_(size|space) to be toku_mempool_*_size
Rename dmt_size->num_klpairs, dmt_iterate*->iterate*