ISPN-2786 Add test to verify marshalling thread local leaks

* Created a test that can detect thread local leaks with different
configurations.
* Thread locals can only be removed if the cache or cache manager
references are nulled, even after shutdown.
* At shutdown, Infinispan can only remove thread locals created
for the thread that's shutting down Infinispan, which is not really
useful.
* Thanks Dan for enhancing the thread local leak test.