Fixes #4745: Move r32077 to v1.8 branch (save power during finalize)

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r32077 [[BR]]
Per the developer conference, let's be a little nicer during MPI_Finalize and ease up on the cpu by inserting usleep into the loop over opal_progress while waiting for the RTE barrier to complete. This is a non-performant area of the code, and while most codes may call finalize at close-to-similar times, there are some that may choose to have one or more procs continue to perform some work prior to finalizing.

So save a little power while we are waiting.

cmr=v1.8.2:reviewer=jladd:subject=save power during finalize