[JENKINS-38514] Retain CauseOfBlockage from JobOffer (#2651)

* Converted to JenkinsRule.

* Improved messages from Node.canTake.

* [FIXED JENKINS-38514] BuildableItem needs to retain information from JobOffer about why it is neither blocked nor building.

* Converted to JenkinsRule.

* Found an existing usage of BecauseNodeIsNotAcceptingTasks.

* Original JENKINS-6598 test was checking behavior we want amended by JENKINS-38514.

* Ensure that a BuildableItem which is simply waiting for a free executor reports that as its CauseOfBlockage.

* Review comments from @oleg-nenashev.