Confirm Signatures and Finalizing UTXO positions about plasma-mvp-sidechain

[AdityaSripal]

To verify a UTXO being spent is valid:
If it is a deposit:
Form the corresponding transaction that created the deposit and check if hash matches with rootchain. Also check that this transaction is not in the Spent Deposit store.

If it is not a deposit:
Check that the UTXO exists in the (Position => UTXO) UTXO store.

For both cases:
Verify that the signature matches address in Owner field and denom matches claimed amount

[colin-axner]

#25