Refactor Util.isTokenUtxo()

[christroutner]

The scope of this Issue is to refactor Util.isTokenUtxos() to match the refactoring done to tokenUtxoDetails() in PR #86. The new behavior of this function will return confident false or true. It will only return null if SLPDB has not processed the UTXO yet and has not yet made a determination of its validity.

The workflow for this refactor is as follows:

• For each UTXO, retrieve the raw transaction data from the full node.
• If there is no OP_RETURN in the TX, then the UTXO can be marked as false.
• If there is an OP_RETURN, attempt to decode it. If it can't be decoded, then the UTXO can be marked as false.
• If the OP_RETURN can be decoded as an SLP transaction, then the txid is validated with SLPDB.
• If SLPDB returns null, it means it has not yet processed the utxo. The UTXO should be marked as null to communicate this.
• Otherwise the UTXO can be marked true