TODO

features
  method to copy a MemStore tree into LMDB, replacing witness with trees when possible
  pruning
    methods to perform pruning
    sync to/from pruned trees
  proofs suitable for deletion (uses witnessLeafs instead of witnesses where necessary)
    ? WitnessBranch: only advantage is it reduces needed strands by 1
  de-dup trees using diff functionality
  ? changeable hash function

docs
  graphs of sync benchmarks

bugs
  !? iterator bug with hashed keys
  exporting proof of 0 keys should return a root witness
  in garbage collection, never collect the highest-ID node in the DB, to prevent ID re-use
  ? clean-up key/keyHash terminology
  ? resource limits on proof sizes when verifying (setMaxProofCmds?)

opt
  ? is there a way to avoid free()ing the std::map entries in putAux, so it can be free()d later, after LMDB write-lock released?
  ? tree compaction to ensure nodeIds are sequential
  ? flatmaps instead of std::map, ie in the update change-set

tests
  tests for diff, mergeProof, gc
  refactor test lib, multiple files
  ability to run a single test/file during dev