IH stage speedup and lmdb custom comparators support (#1080)

* etl.Loader - allow use of custom comparator

* log timing

* try now

* try now

* more performance

* etl.Loader - allow use of custom comparator

* working version

* simplify IH cursor

* clean

* squash

* squash

* squash

* squash

* squash

* squash

* squash

* clean

* add only unwind support

* squash

* squash

* clean

* fix test

* clean

* clean

* clean