Add min mempool estimate for feerate updates on anchor channels

fuzz/src/chanmon_consistency.rs

@@ -78,7 +78,7 @@ impl FeeEstimator for FuzzEstimator {
 		// Background feerate which is <= the minimum Normal feerate.
 		match conf_target {
 			ConfirmationTarget::HighPriority => MAX_FEE,
-			ConfirmationTarget::Background => 253,
+			ConfirmationTarget::Background|ConfirmationTarget::MempoolMinimum => 253,
 			ConfirmationTarget::Normal => cmp::min(self.ret_val.load(atomic::Ordering::Acquire), MAX_FEE),
 		}
 	}

lightning/src/chain/chaininterface.rs

@@ -35,15 +35,24 @@ pub trait BroadcasterInterface {
 	fn broadcast_transactions(&self, txs: &[&Transaction]);
 }
 
-/// An enum that represents the speed at which we want a transaction to confirm used for feerate
+/// An enum that represents the priority at which we want a transaction to confirm used for feerate
 /// estimation.
 #[derive(Clone, Copy, Debug, Hash, PartialEq, Eq)]
 pub enum ConfirmationTarget {
-	/// We are happy with this transaction confirming slowly when feerate drops some.
+	/// We'd like a transaction to confirm in the future, but don't want to commit most of the fees
+	/// required to do so yet. The remaining fees will come via a Child-Pays-For-Parent (CPFP) fee
+	/// bump of the transaction.
+	///
+	/// The feerate returned should be the absolute minimum feerate required to enter most node
+	/// mempools across the network. Note that if you are not able to obtain this feerate estimate,
+	/// you should likely use the furthest-out estimate allowed by your fee estimator.
+	MempoolMinimum,
+	/// We are happy with a transaction confirming slowly, at least within a day or so worth of
+	/// blocks.
 	Background,
-	/// We'd like this transaction to confirm without major delay, but 12-18 blocks is fine.
+	/// We'd like a transaction to confirm without major delayed, i.e., within the next 12-24 blocks.
 	Normal,
-	/// We'd like this transaction to confirm in the next few blocks.
+	/// We'd like a transaction to confirm in the next few blocks.
 	HighPriority,
 }