Never delete spent utxos from the database

Never delete spent utxos from the database

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