Big threading code cleanup.

- The CollectionOfStructure's are now backed by an ArrayList, instead of
  a thread safe Vector. Iterators use a read only synchronised view.

- The grizzly server now only starts 1 thread by default, this can be
  increased by the configuration option maxThreadsRestWorkers and
  maxThreadsRestKernel for the selector threads. Fix #5

- The executors now use a fixed thread pool for execution. Fix #4.