rocksdb : add byte helpers and use atomic batches (#526)

* methods use atomic write/read batches, remove flush

* update test to teardown and use async commit

* tree.get can be passed batch

* add getByteRange helper

* proof helpers all support batches

* remove stale code

* update to new storage api

* update to use peakLastTreeNode

* use deleteRange for truncations

* reopen a tree from db storage

* remove cache

* right span available nodes

* skip cache test

* skip all but merkle tree tests

* update for batch api

* rename to dbBatch for clarity

* standard

* kill stale code

* rename to read/writeBatch

* truncation should delete parent nodes