Update RPC implementation to handle message UUIDs correctly

[PLeVasseur]

Highlighting a comment I made on a previous PR:

I think this RPC flow still needs some thought though.

We should not be creating a new UUIDBuilder here and getting the reqid from it. I believe this is where the UUIDBuilder that's been constructed already for the uE sending the request should be used.

As is stated over here in the UUID spec, the rand_b portion of the UUID is used to uniquely identify the uE, thus it should be "stable" and thus we should use the UUIDBuilder already associated with this uE.

I suppose that may mean that on construction, you'd need to construct a UUIDBuilder and then use it within RpcClient::invoke_method().

WDYT?

[evshary]

Make sense. I'll see how to make the changes

[evshary]

Close with #24