Update Snapshot & Root approach

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.

circomproofs.go

@@ -63,7 +63,10 @@ func (t *Tree) GenerateCircomVerifierProof(k []byte) (*CircomVerifierProof, erro
 		return nil, err
 	}
 	var cp CircomVerifierProof
-	cp.Root = t.Root()
+	cp.Root, err = t.Root()
+	if err != nil {
+		return nil, err
+	}
 	s, err := UnpackSiblings(t.hashFunction, siblings)
 	if err != nil {
 		return nil, err