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Observe group stats

See merge request !11

Add the possibility to register multiple membership update callbacks

See merge request !14

faster load

See merge request !12

fix serialization of ssg group config

See merge request !10

Pmix bugfixes

See merge request !8

I wasn't using 'pcc' so PMIx libraries did not show up at configure
time.  Furthermore, if ssg-calling programs are not using pcc,
had to update pkg-config to tell them about a pmix dependency

start and end ranks should be inclusive

Portability fix

See merge request sds/ssg!7

thanks to recent addition of margo_push_prefinalize_callback, the group
creating process can now exit cleanly.

closes #11

Test finalize callback

See merge request sds/ssg!6

Once in a while an RPC will take 2+ seconds on theta.  stupid theta...