[JEP-222] WebSocket support (#4369)

* Playing with WebSocket connections.

* Pluggable handlers.

* errorWithoutStack

* Server-side keepalive pings.

* Redesigned to work as an HttpResponse.

* Unused dep.

* Comment on JnlpSlaveRestarterInstaller.

* Sketch of WebSocket-based endpoint for JNLPLauncher.

* Comment.

* Test enhancement.

* Making it more obvious from JNLPLauncherTest where remoting.jar is being loaded from.

* Reworked protocol to negotiate remote capabilities.

* Unhelpful comment.

* Simplifying handshake to use HTTP headers.

* Moving code into top-level classes and otherwise prettifying.

* Linking to upstream PRs.

* Timestamped snapshot + incremental.

* Picking up incrementalified build of winstone.

* Working around https://github.com/kohsuke/access-modifier/pull/17.

* Finally have an incrementalified build of remoting.

* Use -webSocket option.

* Handling some close and error methods.

* Sending X-Remoting-Minimum-Version.

* If hudson.remoting.jnlp.Main._main fails, show the error.

* Comment.

* GUI analogue of https://github.com/jenkinsci/remoting/pull/357/commits/86cea5b83fd2fc93a9fc10896962eb42c95f80a0.

* Missing since tags.

* Capitalization.

* WebSockets.isSupported

* Rather than hiding JNLPLauncher.DescriptorImpl when the TCP port is disabled, always display it, but show form validation appropriate to WebSocket or TCP mode.

* Form validation fixes.

* Minor test improvements.

* Reworked WebSocketAgents to be compatible with JnlpAgentReceiver.

* Removing WebSocketSession.keepAlive in favor of a global setting applicable to all services.

* Add a -webSocket option to the Jenkins CLI.

* Using a snapshot deployment of Remoting, pending INFRA-2379.

* After #3838 there is no reason to recheck authentication after parsing CLICommand arguments.

* Advertise the -webSocket option.

* Adapt to newer HtmlUnit.

* https://github.com/HtmlUnit/htmlunit/pull/29 seems to have been incompatible.
WebClient.addRequestHeader will no longer override a header in a WebRequest,
such as when the same WebClient/WebRequest was previously used with different headers.

* Tracked down a behavioral change in passing through URL-encoded path characters.
https://github.com/HtmlUnit/htmlunit/commit/2c4956863420e4baef9d3d8c23ec0577ec64d2bf picks up
https://github.com/apache/httpcomponents-client/commit/8c04c6ae5e5ba1432e40684428338ce68431766b which is the actual change.

* Disabled TCP port does not matter in WebSocket mode.

* Shade dependencies needed for jenkins-cli.jar.

* Help text edit.

* https://github.com/jenkinsci/winstone/pull/79 was released as 5.5.

* https://github.com/jenkinsci/jenkins-test-harness/pull/183 released as 2.59.

* https://github.com/jenkinsci/jenkins/pull/4387#discussion_r362696234

* Bumping remoting to a new deployed snapshot.

* Need https://github.com/jenkinsci/winstone/pull/86.

* https://github.com/jenkinsci/winstone/pull/86 released as 5.6.

* Introduced constant for X-Remoting-Minimum-Version.

* s/slave/agent/ in GUI

* Removing in-JVM test, as it was no longer useful after introducing shading anyway.

* Bump.

* No need to check for anonymous CONNECT here.

* s/jenkins.slaves/jenkins.agents/g for new code.

* Restrict the diagnostic endpoint to administrators.

* Fixed Javadoc import after package move.

* https://github.com/jenkinsci/remoting/pull/357 released as 4.0.