contractcourt+rpc: use new FetchChannel method instead of scanning

In this commit, we modify areas where we need to force close a channel
to use the new FetchChannel method instead of manually scanning. This
dramatically reduces the CPU usage when doing things like closing a
large number of channels within lnd.

contractcourt/chain_arbitrator.go

@@ -224,23 +224,10 @@ func newActiveChannelArbitrator(channel *channeldb.OpenChannel,
 			// With the channels fetched, attempt to locate
 			// the target channel according to its channel
 			// point.
-			dbChannels, err := c.chanSource.FetchAllChannels()
+			channel, err := c.chanSource.FetchChannel(chanPoint)
 			if err != nil {
 				return nil, err
 			}
-			var channel *channeldb.OpenChannel
-			for _, dbChannel := range dbChannels {
-				if dbChannel.FundingOutpoint == chanPoint {
-					channel = dbChannel
-					break
-				}
-			}
-
-			// If the channel cannot be located, then we
-			// exit with an error to the channel.
-			if channel == nil {
-				return nil, fmt.Errorf("unable to find channel")
-			}
 
 			chanMachine, err := lnwallet.NewLightningChannel(
 				c.cfg.Signer, c.cfg.PreimageDB, channel, nil,