lnwallet: rewrite channelState to bool for clarity

Over the last few commits we have systematically eliminated all but
two states. This allows us to replace it with a boolean to encode
the two remaining states. We would like to be able to eliminate this
field entirely, but doing so requires being able to prove that the
concurrent request block is necessary. This is more difficult and
will be left to future commits.

lnwallet/channel_test.go

@@ -2147,8 +2147,8 @@ func TestCooperativeCloseDustAdherence(t *testing.T) {
 	}
 
 	resetChannelState := func() {
-		aliceChannel.status = channelOpen
-		bobChannel.status = channelOpen
+		aliceChannel.isClosed = false
+		bobChannel.isClosed = false
 	}
 
 	setBalances := func(aliceBalance, bobBalance lnwire.MilliSatoshi) {