Case tree empty, AddBatch was 10.95x times faster than without AddBatch
	nCPU: 4, nLeafs: 1024, hash: Poseidon, db: memory
	dbgStats(hash: 2.047k, dbGet: 1, dbPut: 2.049k)
Case tree not empty w/ few leafs, AddBatch was 7.28x times faster than without AddBatch
	nCPU: 4, nLeafs: 1024, hash: Poseidon, db: memory
	dbgStats(hash: 2.047k, dbGet: 198, dbPut: 2.049k)
Case tree not empty w/ enough leafs, AddBatch was 5.94x times faster than without AddBatch
	nCPU: 4, nLeafs: 1024, hash: Poseidon, db: memory
	dbgStats(hash: 2.047k, dbGet: 1.000k, dbPut: 2.049k)
Case tree not empty, AddBatch was 9.27x times faster than without AddBatch
	nCPU: 4, nLeafs: 4096, hash: Poseidon, db: memory
	dbgStats(hash: 8.191k, dbGet: 1.800k, dbPut: 8.193k)
Case tree not empty & unbalanced, AddBatch was 10.67x times faster than without AddBatch
	nCPU: 4, nLeafs: 4096, hash: Poseidon, db: memory
	dbgStats(hash: 10.409k, dbGet: 2.668k, dbPut: 10.861k)
TestAddBatchBench: nCPU: 4, nLeafs: 50000, hash: Blake2b, db: badgerdb
	Add loop:	10.10829114s
	AddBatch:	732.030263ms
		dbgStats(hash: 122.518k, dbGet: 1, dbPut: 122.520k)
TestDbgStats
	add in loop in emptyTree dbgStats(hash: 141.721k, dbGet: 134.596k, dbPut: 161.721k)
	addbatch caseEmptyTree dbgStats(hash: 24.402k, dbGet: 1, dbPut: 24.404k)
	addbatch caseNotEmptyTree dbgStats(hash: 26.868k, dbGet: 2.468k, dbPut: 26.872k)

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