WIP: added group hash

[shanedsnyder]

In GitLab by @mdorier on Jan 9, 2021, 16:25

This PR adds a "group hash", computed and maintained up to date as follows:

hash = 0
foreach member_id in group {
    hash = xor(hash, member_id)
}

It is a preliminary addition to enable services/processes to check whether they have the same view of a group, and could be used in a future ssg_group_refresh function to issue an RDMA operation only if the hash differs.

I'm marking this PR as a WIP; it should be rebased onto master once master gets updated with the branch that adds better error codes.