multi: link quit can interrupt commitment signing

In this commit, we make sig job handling when singing a next commitment
non-blocking by allowing the shutdown of a channel link to prevent
further waiting on sig jobs by the channel state machine. This addresses
possible cases where the aux signer may be shut down via a separate quit
signal, so the state machine could block indefinitely on receiving an
update on a sig job.

lnwallet/transactions_test.go

@@ -357,7 +357,7 @@ func testVectors(t *testing.T, chanType channeldb.ChannelType, test testCase) {
 
 	// Execute commit dance to arrive at the point where the local node has
 	// received the test commitment and the remote signature.
-	localNewCommit, err := localChannel.SignNextCommitment()
+	localNewCommit, err := localChannel.SignNextCommitment(ctxb)
 	require.NoError(t, err, "local unable to sign commitment")
 
 	err = remoteChannel.ReceiveNewCommitment(localNewCommit.CommitSigs)
@@ -369,7 +369,7 @@ func testVectors(t *testing.T, chanType channeldb.ChannelType, test testCase) {
 	_, _, err = localChannel.ReceiveRevocation(revMsg)
 	require.NoError(t, err)
 
-	remoteNewCommit, err := remoteChannel.SignNextCommitment()
+	remoteNewCommit, err := remoteChannel.SignNextCommitment(ctxb)
 	require.NoError(t, err)
 
 	require.Equal(