Fixes #4567: Move r31526 to v1.8 branch (btl/ugni: always buffer sendi fragments)

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r31526 [[BR]]
btl/ugni: always buffer sendi fragments

This commit will improve the message rate when using the sendi function
by not waiting for the send to get to the remote process.

cmr=v1.8.2:reviewer=ompi-rm1.8
Fixes #4527: Move r31416, r31424 to v1.8 branch (Per comments from Jeff makes)

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r31416 [[BR]]
Silence warnings identified by the clang static analyzer in the
communicator code.

Many of the warnings were false warnings. These were silenced by
adding the appropriate asserts. Other warnings identified some
potential issues in error paths that should now be resolved.

cmr=v1.8.1:reviewer=jsquyres

r31424 [[BR]]
Per comments from Jeff makes some changes to the communicator changes.

Changed:

 - Use ompi_mpi_group_null instead of MPI_GROUP_NULL.

 - Asserts don't always quiet the clang static analyser. Change them to
   ifs to really quite the warnings.

cmr=v1.8.1:ticket=#4527:reviewer=jsquyres
Fixes #4585: Move r31574 to v1.8 branch (mtl/psm: do not limit PSM)

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r31574 [[BR]]
mtl/psm: do not limit PSM to 8191 context ids

The old default context id maximum was committed to the trunk in
2006. After some discussion with Intel it appears this is restricting
the mtl to an arbirarly small number of communicators. Increasing the
default to allow up to 2^16 - 1 context ids.

Refs #4574

cmr=v1.8.2