Noting that there is a potentially valid argument for allowing this method to work without the singleton...

- But the type of tests here would be very restricted in what they could safely do anyway, so they would largely be restricted to verifying data migration during xml loading

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

- 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.

[JENKINS-33068] AsyncPeriodicWork/AsyncAperiodicWork log files should be appended and rotated not overwritten each execution

Fixed possible ArrayIndexOutOfBoundsException in
hudson.model.Fingerprint.RangeSet.fromString(...)

(cherry picked from commit d8a13ac0)

(cherry picked from commit 758c0ced)

This change adds code to check that the user isn't attempting to rename
an existing node with the name of another existing node.  Previous to
this change, such rename operations would succeed and would corrupt node
configuration data.

(cherry picked from commit 16d6429c)

Now that the item list is obtained lock-free the cache is no longer needed.
Besides, the cache could be returning invalid results for requests with different authorization to the one that cached the result (in any case this invalid result is transient).

Another fix for the future node_monitors package extraction.

This time, a simple import removal.

This modification is one of the preliminary steps to the full plugin
extraction later (see JIRA above).
Basically reverts a8c3a031.

This change adds code to check that the user isn't attempting to rename
an existing node with the name of another existing node.  Previous to
this change, such rename operations would succeed and would corrupt node
configuration data.

[FIXED JENKINS-31954] Stack overflow reconfiguring some settings where Descriptor.newInstance is gratuitously overridden.

online-node covered by test-cases

This caused a NullPointerException since newInstance was then called reëntrantly.
Adjusting NewInstanceBindInterceptor.processed handling to behave correctly in this case.

NodePropertyTest.invisibleProperty demonstrates that the fallback for onConvert must be onConvert, not instantiate, of the delegate.