This commit changes the way ompi_proc_t's are retained/released by
ompi_group_t's. Before this change ompi_proc_t's were retained once
for the group and then once for each retain of a group. This method
adds unnecessary overhead (need to traverse the group list each time
the group is retained) and causes problems when using an async
add_procs.
Signed-off-by: Nathan Hjelm <hjelmn@lanl.gov>