channeldb+routing: add tx parameter

Adds an optional tx parameter to ForAllOutgoingChannels and FetchChannel
so that data can be queried within the context of an existing database
transaction.

contractcourt/chain_arbitrator.go

@@ -258,7 +258,7 @@ func (a *arbChannel) NewAnchorResolutions() (*lnwallet.AnchorResolutions,
 	// same instance that is used by the link.
 	chanPoint := a.channel.FundingOutpoint
 
-	channel, err := a.c.chanSource.FetchChannel(chanPoint)
+	channel, err := a.c.chanSource.FetchChannel(nil, chanPoint)
 	if err != nil {
 		return nil, err
 	}
@@ -301,7 +301,7 @@ func (a *arbChannel) ForceCloseChan() (*lnwallet.LocalForceCloseSummary, error)
 	// Now that we know the link can't mutate the channel
 	// state, we'll read the channel from disk the target
 	// channel according to its channel point.
-	channel, err := a.c.chanSource.FetchChannel(chanPoint)
+	channel, err := a.c.chanSource.FetchChannel(nil, chanPoint)
 	if err != nil {
 		return nil, err
 	}