[FIXED JENKINS-21780] Revised fix to make cache work properly without introducing a new remote call to find hash codes.

[FIXED JENKINS-21958] FilePath.copyRecursiveTo given a scanner including a symlink but not any regular file in the same directory would neglect to create the parent directory on the destination and thus not copy the link.
(Util.createSymlink is partly to blame for not throwing a descriptive IOException when it fails to create a link,
but this is its historical behavior needed for compatibility.
copyRecursiveTo also continues to suppress error text from this method because there is nowhere for it to go.
Perhaps need a new variant of createSymlink that is guaranteed to either have created the link or throw an exception.)

[FIXED JENKINS-16332] Ensure that User records are saved when the fullName is not recoverable from the id.
Otherwise User.get(id).getFullName() after restarting Jenkins will produce the wrong result.

[FIXED JENKINS-12124] Occasionally errors loading plugin classes since it is expected that findClass (and findLoadedClass) are called under synchronization.
The problem was masked by a blind assumption that an InvocationTargetException was in fact wrapping a ClassNotFoundException.
Many thanks to @gotwarlost for demonstrating how to reproduce the problem and diagnosing the cause.

https://trello.com/c/NvZt4WXu/15-rawhtmlmarkupformatter

Prevents multiple FingerprintActions.

[FIXED JENKINS-21023] Rewrote WorkspaceCleanupThread to determine where workspaces for known items should be, rather than looking for directories that look like they might be workspaces.
One key advantage is that this correctly handles the new default workspace location on the master.
Another is that this allows folders to be correctly skipped, and jobs in folders to be correctly deleted.
A subtler advantage would be handling of configured nondefault workspace locations, and compatibility with WorkspaceLocator.
(Orphaned workspaces from deleted jobs are not removed, but these were not removed before either.)

If it's significant enough to introduce regressions, it's significant enough for the changelog.