Revert "coll/basic: fix segmentation fault in neighborhood collectives if the degree"

This reverts commit 9c788ff9400960823637dc0eebb3a8640414fa05.
coll/basic: fix segmentation fault in neighborhood collectives if the degree
of the topology is higher than the communicator size

It is possible to have a topology degree higher than the size of the communicator.
For example, a periodic cartesian communicator on MPI_COMM_SELF. This will leave
the neighborhood collectives with a request buffer that is too small.

This commits introduces a semantic change :
from now, c_topo must be set before invoking coll_select
* comment on communicator creation in mca_topo_base_dist_graph_create(...)
* use accesors to retrieve topo info
This commit contains new dstore component sm which is used for communication between pmix server and clients at the same node via shared memory.
Merge pull request #263 from elenash/master

dstore sm component implementing shared memory database for pmix client/server communication
fix incorrect mca param registration in latest commit
Silence some warnings and remove debug output
Merge pull request #227 from ggouaillardet/rfc/coll_basic_neighbor

RFC/coll basic neighbor