Add dbgStats metrics to analyze number of Hashes, db Gets, and db Puts.

Current benchmarks:
```
CASE A, AddBatch was 8.841700 times faster than without AddBatch
	nCPU: 4, nLeafs: 1024, hash: Poseidon, db: memory
	dbgStats(hash: 2.044k, dbGet: 1, dbPut: 2.049k)
CASE B, AddBatch was 7.678766 times faster than without AddBatch
	nCPU: 4, nLeafs: 1024, hash: Poseidon, db: memory
	dbgStats(hash: 2.044k, dbGet: 199, dbPut: 2.049k)
CASE C, AddBatch was 8.401087 times faster than without AddBatch
	nCPU: 4, nLeafs: 1024, hash: Poseidon, db: memory
	dbgStats(hash: 2.044k, dbGet: 207, dbPut: 2.049k)
CASE D, AddBatch was 2.466346 times faster than without AddBatch
	nCPU: 4, nLeafs: 4096, hash: Poseidon, db: memory
	dbgStats(hash: 33.884k, dbGet: 30.697k, dbPut: 33.889k)
CASE E, AddBatch was 1.958160 times faster than without AddBatch
	nCPU: 4, nLeafs: 4096, hash: Poseidon, db: memory
	dbgStats(hash: 41.419k, dbGet: 37.558k, dbPut: 41.874k)
TestAddBatchBench: nCPU: 4, nLeafs: 50000, hash: Blake2b, db: leveldb
	Add loop:	10.089858449s
		dbgStats(hash: 825.285k, dbGet: 788.869k, dbPut: 925.285k)
	AddBatch:	904.647829ms
		dbgStats(hash: 122.458k, dbGet: 1, dbPut: 122.463k)
TestDbgStats
	add in loop    dbgStats(hash: 141.915k, dbGet: 134.602k, dbPut: 161.915k)
	addbatch caseA dbgStats(hash: 24.528k, dbGet: 1, dbPut: 24.533k)
	addbatch caseD dbgStats(hash: 115.506k, dbGet: 97.482k, dbPut: 115.516k)
```

- AddBatch use Virtual Tree for cases A,B,C
- ImportDump use AddBatch instead of adding one by one
- Reorg & add more virtual tree tests

VirtualTree (vt) computes a tree without computing any hash. With the
idea of once all the leafs are placed in their positions, the hashes can
be computed, avoiding computing a node hash more than one time.