Merge bitcoindevkit/bdk#515: Never delete spent utxos from the database

f2f0efc0b3367abf40e33b84fc577fb11f625b04 Never delete spent utxos from the database (Daniela Brozzoni)

Pull request description:

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  ### Description

  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 in mempool utxos, effectively excluding them from the coin selection and balance calculation
  Fixes #414

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

  #### New Features:

  * [x] I've added tests for the new feature
  * [x] I've updated `CHANGELOG.md`
  * [x] I'm linking the issue being fixed by this PR

ACKs for top commit:
  afilini:
    Re-ACK f2f0efc

Tree-SHA512: 7984b69c56b3b12746c2301072d637dea1346730aa132f1ea67c5bd8bf685bd557fc7cbdc2fc1185e63c71db07d36cd979514921b2c83c55737cd4b96102377e