sweep: add monitor loop to TxPublisher

This commit finishes the implementation of `TxPublisher` by adding the
monitor process. Whenever a new block arrives, the publisher will check
all its monitored records and attempt fee bumping them if necessary.

contractcourt/utxonursery.go

@@ -1406,6 +1406,10 @@ func (k *kidOutput) ConfHeight() uint32 {
 	return k.confHeight
 }
 
+func (k *kidOutput) RequiredLockTime() (uint32, bool) {
+	return k.absoluteMaturity, k.absoluteMaturity > 0
+}
+
 // Encode converts a KidOutput struct into a form suitable for on-disk database
 // storage. Note that the signDescriptor struct field is included so that the
 // output's witness can be generated by createSweepTx() when the output becomes