add raft support to SSG

[shanedsnyder]

In GitLab by @shanedsnyder on Jul 3, 2017, 16:36

SSG+SWIM will allow for us to provide weakly-consistent group membership views across an SSG group, but that might not be enough for all use cases -- some services are likely going to require strict ordering of group membership changes.

We should investigate how the RAFT protocol can be integrated into SSG as a consensus component responsible for ordering membership updates.

Our Intel friends have already forked and improved a publicly available Raft implementation, with their fork available here: https://github.com/daos-stack/raft