Since InterceptorUtil is a singleton, use ConcurrentSkipListSet for the
ClassloaderRequired Sets. We are not using any bulk operations or
iterators, so ConcurrentSkipListSet is appropriate.
Properly sealed constructor
Made InterceptorUtil utilize the initialization-on-demand holder idiom
to prevent a race condition on initialization.
Merge pull request #76 from abaxter-rh4u/master

Corrected possible race conditions surrounding InterceptorUtil's use in a multi-threaded environment