Signed-off-by: NNicolas De Loof <nicolas.deloof@gmail.com>

[JENKINS-34767] - Prevent RSS ID collisions for items with same name in different folders

- Code that is running from a plugin and on the master's JVM is guaranteed to never get null from this method (any cases where you do get null are bugs in core)
- Code that is running from a plugin and on a remote JVM should never be allowed to load the Jenkins class in their classloader, so should never use Jenkins.getInstance()... we are annotating the method with @Nullable so that such code can have some evolution time
- Code that is running in core and on one of two special paths should use the Jenkins.getInstanceOrNull() method so that the UI can be presented to users before the singleton has been instantiated / after the singleton has been destroyed
- The remaining 95% of uses in core (and 100% of uses in plugins) can safely assume that the instance is never null

This reverts commit bb7c8fce.

Closes #2090, I'll redo this as a PR... though if that PR is subject to multiple rounds of review before being merged then I will take that of evidence of the exact problem that committing directly was supposed to resolve... namely exponentially multiplying the effort required to make actual improvements to the code base.

- Only ever called after extensions are on the classpath and before termination

- It is never too late to do the right thing.
- The vast majority of usages of `Jenkins.getInstance()` in core currently assume that its return value is non-null
- This commit changes those that are written to correctly check for non-null values will call `Jenkins.getInstanceOrNull()`
- We deprecate the `Jenkins.getActiveInstance()` madness
- I checked with @kohsuke who said not to bother with a PR and just commit this strongly opinionated change direct to master as a PR will just degrade into a bikeshedding.

* moved to jenkins.util.SystemProperties
* consistent naming getString/getInteger/getBoolean
* improved code formatting
Improved JavaDoc

Accomplished by centralizing calls to System.getProperty(String) and related into new file SystemProperties.java.
There, we first check for existence of system property; if not, we look for property in context.xml
This is done for "application" properties (like hudson.DNSMultiCast.disabled) but not for java properties (like user.name)

In addition to cluttering the namespace, unused imports generate
compiler warnings, introducing noise and obscuring more important
compiler warnings.  This change removes them.

(cherry picked from commit 20bd4e30)

return an empty list when called with maxLines = 0
added unit test for getLog(0) returning an empty list

(cherry picked from commit 75f58103)

return an empty list when called with maxLines = 0
added unit test for getLog(0) returning an empty list

[JENKINS-26698] Make sure BuildExecution.cleanUp always calls super, to release the workspace lease.
Also stop closing the build log in Run.handleFatalBuildProblem; we want to see any subsequent messages too.
(cherry picked from commit 62af87b6)

[FIXED JENKINS-20989] PeepholePermalink RunListenerImpl oncompleted should be triggered before downstream builds are triggered

Allowing correlation/tracking all the way through the lifecycle

(cherry picked from commit dbed3044)

Conflicts:
	core/src/main/java/hudson/model/Run.java

[JENKINS-26698] Make sure BuildExecution.cleanUp always calls super, to release the workspace lease.
Also stop closing the build log in Run.handleFatalBuildProblem; we want to see any subsequent messages too.

fromExternalizableId should just return null, not throw IAE, if the job has since been renamed/move/deleted.
(It could already return null if the run has since been deleted; #1264 failed to mark this properly.)
Reserving IAE for inputs that getExternalizableId could not actually have produced,
and making sure to also throw this in case of non-numeric text after the hash.
Adding Javadoc for both methods.

For example, the following should not cause Run.onLoad itself to fail:
java.lang.NullPointerException
	at org.jenkinsci.plugins.workflow.steps.input.InputAction.onLoad(InputAction.java:29)
	at hudson.model.Run.onLoad(Run.java:342)
	at org.jenkinsci.plugins.workflow.job.WorkflowRun.onLoad(WorkflowRun.java:332)

Assertions are only for cases where you believe the failure of the condition is impossible.
For cases where external code might mistakenly make an incorrect call, throw an exception.

The Run constructors taking a timestamp must not be deprecated.
And the root directory must be created in RunMap.put, not the constructor,
in case the number (and hence directory name) is changed after the constructor completes.

Does not yet deal with migrating historical records.