Remoting should reject RPCRequests over the closing channel

It is a follow-up to JENKINS-45023, where I have added checks for UserRequests in Remoting. It should help with the most of cases, but some requests like remote CommandLaunchers in Jenkins core may still hang because they use RPCRequests which are not filtered.

OTOH some system objects are actually expected to be executed for closing channels (e.g. ClassLoaders), hence it needs additional investigation.