lnwallet+htlcswitch: define expanded NumPendingUpdates

This commit squashes the below operations for a net result where
we have an expanded capability of assessing pending updates. This
is made possible by packing the components into Duals in the prior
commits. We squash the operations to simplify review.

htlcswitch+lnwallet: rename PendingLocalUpdateCount

lnwallet: complete pending update queries API for LightningChannel

lnwallet+htlcswitch: consolidate NumPendingUpdates using ChannelParty

This commit makes the observation that we can cleanly define the
NumPendingUpdates function using a single expression by taking
advantage of the relevant fields being properly packed into Duals.

lnwallet/channel.go

@@ -5357,16 +5357,18 @@ func (lc *LightningChannel) oweCommitment(issuer lntypes.ChannelParty) bool {
 	return oweCommitment
 }
 
-// PendingLocalUpdateCount returns the number of local updates that still need
-// to be applied to the remote commitment tx.
-func (lc *LightningChannel) PendingLocalUpdateCount() uint64 {
+// NumPendingUpdates returns the number of updates originated by whoseUpdates
+// that have not been committed to the *tip* of whoseCommit's commitment chain.
+func (lc *LightningChannel) NumPendingUpdates(whoseUpdates lntypes.ChannelParty,
+	whoseCommit lntypes.ChannelParty) uint64 {
+
 	lc.RLock()
 	defer lc.RUnlock()
 
-	lastRemoteCommit := lc.commitChains.Remote.tip()
+	lastCommit := lc.commitChains.GetForParty(whoseCommit).tip()
+	updateIndex := lc.updateLogs.GetForParty(whoseUpdates).logIndex
 
-	return lc.updateLogs.Local.logIndex -
-		lastRemoteCommit.messageIndices.Local
+	return updateIndex - lastCommit.messageIndices.GetForParty(whoseUpdates)
 }
 
 // RevokeCurrentCommitment revokes the next lowest unrevoked commitment