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`.

lightning/src/util/mod.rs

@@ -21,6 +21,7 @@ pub mod message_signing;
 pub mod invoice;
 pub mod persist;
 pub mod string;
+pub mod sweep;
 pub mod wakers;
 #[cfg(fuzzing)]
 pub mod base32;

lightning/src/util/persist.rs

@@ -65,6 +65,17 @@ pub const SCORER_PERSISTENCE_SECONDARY_NAMESPACE: &str = "";
 /// The key under which the [`WriteableScore`] will be persisted.
 pub const SCORER_PERSISTENCE_KEY: &str = "scorer";
 
+/// The primary namespace under which [`TrackedSpendableOutput`]s will be persisted by [`OutputSweeper`].
+///
+/// [`TrackedSpendableOutput`]: crate::util::sweep::TrackedSpendableOutput
+/// [`OutputSweeper`]: crate::util::sweep::OutputSweeper
+pub const SPENDABLE_OUTPUT_INFO_PERSISTENCE_PRIMARY_NAMESPACE: &str = "spendable_outputs";
+/// The secondary namespace under which [`TrackedSpendableOutput`]s will be persisted by [`OutputSweeper`].
+///
+/// [`TrackedSpendableOutput`]: crate::util::sweep::TrackedSpendableOutput
+/// [`OutputSweeper`]: crate::util::sweep::OutputSweeper
+pub const SPENDABLE_OUTPUT_INFO_PERSISTENCE_SECONDARY_NAMESPACE: &str = "";
+
 /// A sentinel value to be prepended to monitors persisted by the [`MonitorUpdatingPersister`].
 ///
 /// This serves to prevent someone from accidentally loading such monitors (which may need