Several changes:
- Modifications to coll/hcoll component related to the changes in the libhcoll API. 
  Now, hcoll_destroy_context accepts one more parameter that indicates if the context was
  really destroyed as a result of the call. 
  This new "non-blocking" context destruction fixes hang discovered in IMB with mcast enabled. 
- Clean up all the left contexts (if any) on the comm_world destruction. 

fixed by Val, reviewed by miked
cmr=v1.7.4:reviewer=ompi-rm1.7