Don't subclass ThreadLocal to avoid memory leak in NLS

Don't subclass ThreadLocal to avoid memory leak in NLS

These problem usually occur when you subclass ThreadLocal (usually to
implement initialValue). Those classes reference the webapp's
classloader. The ThreadLocal subclass in turn is referenced by each
Thread instance (that's how ThreadLocals are implemented, they have a
"helper-Map" in each Thread instance, so the leak is actually not a tiny
Random instance but the whole webapp's classloader with a bunch of class
definitions and statically referenced parts of the webapp.

Bug: 449321
Change-Id: Ie7a8b0b90e40229e2471202f2a12637b9e0b1d11
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>