lnwallet: add new rebroadcaster interface, use for background tx publish

In this commit, we add a new Rebroadcaster interface to be used for
publishing transactions passively in the background until they've been
confirmed on chain. This is useful if a tx drops out of the mempool, but
then the pool clears down and has more space available to accept the tx
at the current fee level.

lnwallet/config.go

@@ -56,4 +56,9 @@ type Config struct {
 	// NetParams is the set of parameters that tells the wallet which chain
 	// it will be operating on.
 	NetParams chaincfg.Params
+
+	// Rebroadcaster is an optional config param that can be used to
+	// passively rebroadcast transactions in the background until they're
+	// detected as being confirmed.
+	Rebroadcaster Rebroadcaster
 }