lnwallet: eliminate inner-most layer of evil mutateState nonsense

This commit begins the process of moving towards a more principled
means of state tracking. We eliminate the mutateState argument from
processAddEntry and processRemoveEntry and move the responsibility
of mutating said state to the call-sites.

The current call-sites of these functions still have their *own*
mutateState argument which will be eliminated during upcoming commits.
However, following the principle of micro-commits I opted to break
these changes up to make review simpler.

lnwallet/channel_test.go

@@ -9613,15 +9613,25 @@ func TestProcessAddRemoveEntry(t *testing.T) {
 			// update type. Process remove is used for settles,
 			// fails and malformed htlcs.
 			process := processRemoveEntry
+			heightDual := &update.removeCommitHeights
 			if test.updateType == Add {
 				process = processAddEntry
+				heightDual = &update.addCommitHeights
 			}
 
-			process(
-				update, &ourBalance, &theirBalance, nextHeight,
-				test.whoseCommitChain, test.isIncoming,
-				test.mutateState,
-			)
+			if heightDual.GetForParty(test.whoseCommitChain) == 0 {
+				process(
+					update, &ourBalance, &theirBalance,
+					test.isIncoming,
+				)
+
+				if test.mutateState {
+					heightDual.SetForParty(
+						test.whoseCommitChain,
+						nextHeight,
+					)
+				}
+			}
 
 			// Check that balances were updated as expected.
 			if ourBalance != test.ourExpectedBalance {