multi: registration complete MW interceptor msg

In this commit, we change the flow of the rpc middleware registration
a bit. In order to allow a client to add rpc middleware interceptors in
a deterministic order, we now make the server send a "registration
complete" message to the client after compeleting the registration
process so that the client knows when it can go ahead and register the
next client.

docs/release-notes/release-notes-0.15.1.md

@@ -65,6 +65,10 @@
   true so the payment won't be failed unless a terminal error has occurred,
   which is useful for constructing MPP.
 
+* [Add a message to the RPC MW registration 
+  flow](https://github.com/lightningnetwork/lnd/pull/6754) so that the server 
+  can indicate to the client that it has completed the RPC MW registration.
+
 ## Bug Fixes
 
 * Fixed data race found in