Create a comm keyval for hcoll component with delete callback function.
Set comm attribute with keyval.
Wait for pending hcoll module tasks in comm delete callback where PML
still valid on the communicator. safely destroy hcoll context during
hcoll module destructor.

Author: Devendar Bureddy 
reviewed by miked

cmr=v1.7.4:reviewer=ompi-rm1.7