[FIXED JENKINS-16089]

Remember the permalink target as symlink (or simple text file) so that
looking that up doesn't cause the walk of the build history.
I think this is more in line with our general preference of making
$JENKINS_HOME useful (than trying to persist cache into a blackbox.)

Having a general purpose in-memory cache could be useful, so I'll see if
I can add that, too, in a way that allows someone to plug different
backend.