Add LockedHandle class, that locks/unlocks access around ptr

a mutex lock/unlock.

usage:

   std::mutex the_mutex;
   Obj *o = new Obj();
   LockedHandle<Obj> lh(o, &the_mutex);
   o->doSomething();

access to o->doSomething() will be protected by a mutex.

uses a std::mutex by default, but this can be overridden via a second template arg:

   LockedHandle<Obj_TYPE, Mutex_TYPE> lh(obj, &mut);

CPPUnit test is also provided

modified:   GaudiKernel/CMakeLists.txt
new file:   GaudiKernel/GaudiKernel/LockedHandle.h
new file:   GaudiKernel/tests/src/test_LockedHandle.cpp