contractcourt/chain_watcher: handleKnownRemoteState

Similar to what we did for the local state handling, we extract handling
all known remote states we can act on (breach, current, pending state)
into its own method.

Since we want to handle the case where we lost state (both in case of
local and remote close) last, we don't rely on the remote state number
to check which commit we are looking at, but match on TXIDs directly.

channeldb/channel_test.go

@@ -1485,7 +1485,7 @@ func TestBalanceAtHeight(t *testing.T) {
 			targetHeight:          unknownHeight,
 			expectedLocalBalance:  0,
 			expectedRemoteBalance: 0,
-			expectedError:         errLogEntryNotFound,
+			expectedError:         ErrLogEntryNotFound,
 		},
 		{
 			name:                  "height not reached",