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.
in the base).
Correctly deal with persistent requests (they must be always freed when
they are stored in the request array associated with the communicator).
Always use MPI_STATUS_IGNORE for single request waiting functions.
