`del_tx` pulls the TransactionDetails object using
`select_transaction_details_by_txid` method which gets the transaction
details' data with a non-None transaction field even if the
`include_raw` argument is `false`. So it becomes necessary to Set
the transaction field in transactiondetails to None in `del_tx`, when
we make a call to it with `include_raw=false`.
A `is_spent` field is added to LocalUtxo; when a txo is spent we set
this field to true instead of deleting the entire utxo from the
database.
This allows us to create txs double-spending txs already in blockchain.
Listunspent won't return spent utxos, effectively excluding them from the
coin selection and balance calculation
