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 !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 !6

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

Processes in separate address spaces exercise some RMA paths more aggressively,
and also ensure we have RMA credientials set up

otherwise, we could cause deadlock for SSG users if a remote
target has gone completely offline.

if a leaving member fails to forward the request to another
member, shutdown the group locally anyway. the member will
eventually be evicted via fault detection. we can always come
back and add more retry logic here.

if the leave caller is the last group member, do not attempt
to forward a leave request to anyone