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.

Circom Verifier Proofs allow to verify through a zkSNARK proof the
inclusion/exclusion of a leaf in a tree. This commit adds the needed
code in go to generate the circuit inputs for a CircomVerifierProof.