[JENKINS-38867] Optimize performance of Actionable.getAllActions (#2582)

* Optimize Actionable.getAllActions.

* Also need to invalidate the cache when new plugins are installed.

* Various improvements to TransientActionFactory caching.
· Move the cache code to TransientActionFactory itself, for better encapsulation.
· Optimize getAction(Class) to not need to call getAllActions; avoids copying lists, and can avoid calling TransientActionFactory at all.
· Ensure that we maintain a separate cache per ExtensionList instance, so that static state is not leaked across Jenkins restarts.

* Updated TransientActionFactory to specify what kinds of actions it could produce.

* It turns out that changing type parameters for an extension, while generally binary-compatible, breaks reflective code in Jenkins and so this is not an option.
… hudson.ExtensionFinder$GuiceFinder$SezpozModule configure
WARNING: Failed to load com.cloudbees.hudson.plugins.folder.relocate.RelocationAction$TransientActionFactoryImpl
java.lang.LinkageError: Failed to resolve class com.cloudbees.hudson.plugins.folder.relocate.RelocationAction$TransientActionFactoryImpl
	at hudson.ExtensionFinder$GuiceFinder$SezpozModule.resolve(ExtensionFinder.java:489)
	at hudson.ExtensionFinder$GuiceFinder$SezpozModule.configure(ExtensionFinder.java:506)
	at …
	at hudson.ExtensionFinder$GuiceFinder.<init>(ExtensionFinder.java:280)
	at …
	at hudson.ClassicPluginStrategy.findComponents(ClassicPluginStrategy.java:472)
	at hudson.ExtensionList.load(ExtensionList.java:349)
	at hudson.ExtensionList.ensureLoaded(ExtensionList.java:287)
	at hudson.ExtensionList.getComponents(ExtensionList.java:167)
	at jenkins.model.Jenkins$8.onInitMilestoneAttained(Jenkins.java:1082)
	at jenkins.InitReactorRunner$1.onAttained(InitReactorRunner.java:82)
	at org.jvnet.hudson.reactor.ReactorListener$Aggregator.onAttained(ReactorListener.java:104)
	at org.jvnet.hudson.reactor.Reactor$1.run(Reactor.java:176)
	at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:117)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.reflect.MalformedParameterizedTypeException
	at sun.reflect.generics.reflectiveObjects.ParameterizedTypeImpl.validateConstructorArguments(ParameterizedTypeImpl.java:58)
	at sun.reflect.generics.reflectiveObjects.ParameterizedTypeImpl.<init>(ParameterizedTypeImpl.java:51)
	at sun.reflect.generics.reflectiveObjects.ParameterizedTypeImpl.make(ParameterizedTypeImpl.java:92)
	at sun.reflect.generics.factory.CoreReflectionFactory.makeParameterizedType(CoreReflectionFactory.java:105)
	at sun.reflect.generics.visitor.Reifier.visitClassTypeSignature(Reifier.java:140)
	at sun.reflect.generics.tree.ClassTypeSignature.accept(ClassTypeSignature.java:49)
	at sun.reflect.generics.repository.ClassRepository.getSuperclass(ClassRepository.java:90)
	at java.lang.Class.getGenericSuperclass(Class.java:777)
	at hudson.ExtensionFinder$GuiceFinder$SezpozModule.resolve(ExtensionFinder.java:470)
	... 29 more

* Remove actionType override to make for a simpler diff.

* Strengthened test to cover accesses to unrelated context types.

* Strengthening tests in a couple of ways.

* Javadoc improvements suggested by @oleg-nenashev.