Thread safety in SDAI_Application_instance__set

Done by introducing a recursive_mutex in the SDAI_Application_instance__set class.
Removed warning from SDAI_instance__set commit
Added class sdaiSet in cldai library

This class will act as a superclass of few other classes in the same library, hence will contain the common logic of its subclasses.
Made use of sdaiSet

The classes in cldai which were chosen to inherit from sdaiSet were: SDAI_DAObject__set, SDAI_Entity_extent__set and SDAI_Model_contents__list.
Made SDAI__set thread safe

This was done by introducing a mutex in the SDAI__set class.
Added SDAI__set::Remove( SDAI_ptr ) function

The semantics of this function would be similar to SDAI__set::Remove( SDAI__set::Index( SDAI_ptr ) ). The only difference would be that in the new function both the operations are done under a single lock.
Wherever the old Remove( Index( SDAI_ptr ) ) code snippet was found, it was replaced by the new Remove( SDAI_ptr ) function.