GitLab

* init routines take a margo_instace_id rather than HG inputs
* rank resolution happens at init time, so no need for standalone
  ssg_resolve_rank() function
* ssg_lookup is now no longer public and done automatically as
  part of init as well

* refactor ssg_init_config and ssg_init_mpi to reuse common
internal init routine
* update both init routines to resolve self rank and HG address
before returning (this should obviate the need to ever call
ssg_resolve_ranks)
* update test programs according to above