This patch addresses most (if not all) @derbeyn concerns
expressed on #1015. I added checks for the requests allocation
in all functions, ompi_coll_base_free_reqs is called with the
right number of requests, I removed the unnecessary basic_module_comm_t
and use the base_module_comm_t instead, I remove all uses of the
COLL_BASE_BCAST_USE_BLOCKING define, and other minor fixes.
