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/rebroadcaster.go

@@ -0,0 +1,27 @@
+package lnwallet
+
+import (
+	"github.com/btcsuite/btcd/chaincfg/chainhash"
+	"github.com/btcsuite/btcd/wire"
+)
+
+// Rebroadcaster is an abstract rebroadcaster instance that'll continually
+// rebroadcast transactions in the background until they're confirmed.
+type Rebroadcaster interface {
+	// Start launches all goroutines the rebroadcaster needs to operate.
+	Start() error
+
+	// Started returns true if the broadcaster is already active.
+	Started() bool
+
+	// Stop terminates the rebroadcaster and all goroutines it spawned.
+	Stop()
+
+	// Broadcast enqueues a transaction to be rebroadcast until it's been
+	// confirmed.
+	Broadcast(tx *wire.MsgTx) error
+
+	// MarkAsConfirmed marks a transaction as confirmed, so it won't be
+	// rebroadcast.
+	MarkAsConfirmed(txid chainhash.Hash)
+}