fix race condition which can happen on finalize
1. Change in rte api implementation: now comm_world used to do p2p. 
This allows to not worry about other comms being destroyed.

2. added a notification mechanism with a help of which runtime can say libhcoll that RTE api can not be used any longer. 
pass a pointer to a flag, and its size to libhcoll.
The flag changes when the RTE is no longer available. 
Currently this flag is just ompi_mpi_finalized global bool value.

cmr=v1.7.3:reviewer=jladd