multi: refactor select methods within channeldb to use ChannelParty

Also in this commit is a small adjustment to the call-sites to get
the boundaries stitched back together.

contractcourt/channel_arbitrator_test.go

@@ -416,7 +416,9 @@ func createTestChannelArbitrator(t *testing.T, log ArbitratorLog,
 			resolvedChan <- struct{}{}
 			return nil
 		},
-		MarkCommitmentBroadcasted: func(_ *wire.MsgTx, _ bool) error {
+		MarkCommitmentBroadcasted: func(_ *wire.MsgTx,
+			_ lntypes.ChannelParty) error {
+
 			return nil
 		},
 		MarkChannelClosed: func(*channeldb.ChannelCloseSummary,