[JENKINS-36871, JENKINS-37565] JNLP4-connect implementation and Remoting 3 (#2492)

* [JENKINS-36871] Switch to the new JnlpProtocolHandler based implementation

Todo

- [ ] Restore the cookie behaviour (but done right this time)
- [ ] Perhaps investigate issuing clients with TLS certificates (but would require a UI for managing them)

* [JENKINS-36871] License headers and javadocs

* [JENKINS-36871] Restore cookie handling

* [JENKINS-36871] Integrating Agent discovery components

* [JENKINS-36871] Pick up remoting 3.0-SNAPSHOT

* [JENKINS-36871] Pick up newer snapshot

* [JENKINS-36871] Oleg wants to log an exception that cannot happen