lnwallet+contractcourt: publicly export DeriveCommitmentKeys use in chain watcher

In this commit, we consolidate the number of areas where we derive our
commitment keys. Before this commit, the `isOurCommitment` function in
the chain watcher used a custom routine to derive the expected
scripts/keys for our commitment at that height. With the recent changes,
we now have additional logic in `DeriveCommitmentKeys` that wasn't
copied over to this area. As a result, the prior logic would erroneously
detect if it was our commitment that had hit the chain or not.

In this commit, we remove the old custom code, and use
`DeriveCommitmentKeys` wihtin the chain watcher as well. This ensures
that we only need to maintain the key derivation code in a single place,
preventing future bugs of this nature.

lnwallet/wallet.go

@@ -663,11 +663,11 @@ func CreateCommitmentTxns(localBalance, remoteBalance btcutil.Amount,
 	fundingTxIn wire.TxIn,
 	tweaklessCommit bool) (*wire.MsgTx, *wire.MsgTx, error) {
 
-	localCommitmentKeys := deriveCommitmentKeys(
+	localCommitmentKeys := DeriveCommitmentKeys(
 		localCommitPoint, true, tweaklessCommit, ourChanCfg,
 		theirChanCfg,
 	)
-	remoteCommitmentKeys := deriveCommitmentKeys(
+	remoteCommitmentKeys := DeriveCommitmentKeys(
 		remoteCommitPoint, false, tweaklessCommit, ourChanCfg,
 		theirChanCfg,
 	)