TestAddBatchBench
	nCPU: 4, nLeafs: 50000, hash: Blake2b, db: leveldb
		Add loop:	9.304195479s
		AddBatch:	817.590526ms

- Replace naive AddBatch by optimized AddBatch
- Add blake2b hash support
- Expose needed methods for external usage (ReadLeafValue,
ReadIntermediateChilds)
- Return 'value' in GenProof