Instead just keep mempool around a little longer (like pre-refactoring).  This should
reduce large mallocs and remove a memcpy

Refs Tokutek/ft-index#46 Comments, minor renames, reworked bn_data::move_leafentries s.t. splits do not significantly increase memory usage of basement nodes and to be cleaner

now uses dmt iterate_on_range and the dmt builder instead of a for loop

Refs Tokutek/ft-index#46 Killed dmt_functor<> template.  Added dmtwriter_t template parameter to dmt and some documentation

Use dmt to replace omt in bn_data class for storing leafentries.
Optimization for serial inserts and mempool

omt_compress_kvspace.

Isolate mempool and OMT into a new class, bndata.
Remove key from the leafentry.