[JENKINS-13154]

Still seeing some reports of contented lock on annotatedTypes after the initial fix went in May
2012.

This is really puzzling, as the processedTypes should warm up quickly
and include all the relevant classes, so we shouldn't be getting to the
synchronized block (unless WeakHashSet was silently dropping stuff in
1.3.x code.)

This probe will hopefully shed a bit more light on what's going on,
and if we are lucky, XStream 1.4 removing "Weak" from these collections
might resolve the problem entirely (fingers crossed.)