Add `OutputSweeper` utility persisting and sweeping spendable outputs

Add `OutputSweeper` utility persisting and sweeping spendable outputs

We add a `OutputSweeper` utility that allows to track the state of
spendable output descriptors as emitted by `Event::SpendableOutputs`.

To this end, the `OutputSweeper` persists the necessary information in
our `KVStore` and regularly tries to sweep the spendable outputs,
removing them after reaching threshold confirmations, i.e.,
`ANTI_REORG_DELAY`.