SCP-3501: Remove txs from chain index

[ghost]

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[koslambrou]

The implementation looks good. However, I would also add a property test to ensure this works as expected. Here's an idea.

In HandlerSpec, you can add a property test which:

• calls appendBlock
• for all possibles addresses (or at least one address), call utxoSetAtAddress
• Then call getUtxoutFromRef for each utxo
• Verify the result is not Nothing