sweep: handle missing inputs during fee bumping

This commit handles the case when the input is missing during the RBF
process, which could happen when the bumped tx has inputs being spent by
a third party. Normally we should be able to catch the spend early via
the spending notification and never attempt to fee bump the record.
However, due to the possible race between block notification and spend
notification, this cannot be guaranteed. Thus, we need to handle the
case during the RBF when seeing a `ErrMissingInputs`, which can only
happen when the inputs are spent by others.

contractcourt/anchor_resolver.go

@@ -108,7 +108,7 @@ func (c *anchorResolver) Resolve() (ContractResolver, error) {
 
 		// Anchor was swept by someone else. This is possible after the
 		// 16 block csv lock.
-		case sweep.ErrRemoteSpend:
+		case sweep.ErrRemoteSpend, sweep.ErrInputMissing:
 			c.log.Warnf("our anchor spent by someone else")
 			outcome = channeldb.ResolverOutcomeUnclaimed