lnwire: morph CommitSignature into CommitSig

This commit renames and modifies the CommitSignature message to more
closely match the CommitSig message defined within the current set of
draft specifications.

The major change within the new message is that we now longer
explicitly specify the update log index of the remote node that this
signature covers. This is due to the fact the revocation message now
also double as acknowledgements of the remote parties recevied
commitment update messages.

lnwire/message.go

@@ -44,7 +44,7 @@ const (
 	CmdCancelHTLC        = uint32(1300)
 
 	// Commands for modifying commitment transactions.
-	CmdCommitSignature  = uint32(2000)
+	CmdCommitSig    = uint32(2000)
 	CmdRevokeAndAck = uint32(2010)
 
 	// Commands for reporting protocol errors.
@@ -115,8 +115,8 @@ func makeEmptyMessage(command uint32) (Message, error) {
 		msg = &HTLCSettleRequest{}
 	case CmdCancelHTLC:
 		msg = &CancelHTLC{}
-	case CmdCommitSignature:
-		msg = &CommitSignature{}
+	case CmdCommitSig:
+		msg = &CommitSig{}
 	case CmdRevokeAndAck:
 		msg = &RevokeAndAck{}
 	case CmdErrorGeneric: