ThresholdNLeafs defines the threshold number of leafs in the tree that
determines if AddBatch will work in memory or in disk. It is defined
when calling NewTree, and if set to 0 it will work always in disk.

Implement addBatchInDisk for big trees, which does not puts the tree in
memory, and works directly over the db data, parallelizing for each CPU.

Update Snapshot & Root approach to get the root always from the db,
except in the cases that the tree is a snapshot, in which the root will
be in memory.
In this way, when a snapshot is performed and the original tree gets
modifyed, the snapshot will still point to the old root. Also, the root
obtained from the db, uses also the db.ReadTx, so if the root is being
modifyied in the current tx (db.WriteTx), when getting the root it will
be return the lastest version that is in the tx but not yet in the db.

- Update upFromNodes function for unbalanced tree case
- Add AddBatchTestVector2 & 3 with some edge cases
- Add checkRoots test method, which stores the Dump of the tree to file for after-debug