[FIXED JENKINS-23963] Use absolute sidepanel links for computers

[JENKINS-24244] If we are skipping polling due to quietingDown, at least record this fact in the system log.

Trying to make testRemoteKill less prone to random failure (presumably on a heavily loaded machine).

Adapted from #1375 by @daniel-beck: omitting change to User construction, and adding test.

Missed this in previous commit - bad idea making even small changes late at night ;)

[FIXED JENKINS-24358] RunSaveableReference.get can acquire locks, so avoid calling it while holding a lock on ourselves.

Right now, checker is being attached to both onblur and onchange.  The result is when the user changes input, and that input loses focus, the checker is being fired twice.  This can cause issues on non-trivial field changes (such as validating a groovy script).  If there is no change, the checker is still called once due to the onblur.

The checker should only be called if the input is changed.

Added no-store to the Cache-Contorl header

Reference: http://stackoverflow.com/questions/866822/why-both-no-cache-and-no-store-should-be-used-in-http-response
Reference: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9.2

Integrated new dom4j

Integrating dom4j with the fix for java.lang.ClassCastException:
org.dom4j.DocumentFactory cannot be cast to org.dom4j.DocumentFactory

See: https://github.com/stapler/stapler/pull/36

This change improves the handling of errors in hudson.model.Executor, which cause issues like JENKINS-18164.
The change also introduces several annotations in order to prevent further issues.
Signed-off-by: NOleg Nenashev <o.v.nenashev@gmail.com>

[FIXED JENKINS-24095] Ensure only numExecutors exist when reducing