lnrpc+rpcserver: add new field LockedIn for HTLCs

In this commit, we add a new field `LockedIn` on HTLCs so it can be used
to decide whether an HTLC found on the local commitment has been
committed on the remote commitment.

lnrpc/lightning.proto

@@ -1381,6 +1381,13 @@ message HTLC {
 
     // Index identifying the htlc on the forwarding channel.
     uint64 forwarding_htlc_index = 7;
+
+    /*
+    Whether the HTLC is locked in. An HTLC is considered locked in when the
+    remote party has sent us the `revoke_and_ack` to irrevocably commit this
+    HTLC.
+    */
+    bool locked_in = 8;
 }
 
 enum CommitmentType {