Based on current implementation it is faster to use a blocking
send than the non-blocking version. Switch the exchange function
used in the barrier to use the blocking version combined with
the non-blocking version of the receive.
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.
