rename "slave mode"

[ongardie]

I saw this come up in the PRs. Most modern projects avoid the term "slave" since it has unpleasant connotations/denotations. It's probably better to find and replace this early on than to put off potential users and contributors. Moreover, it's not obvious to others what "slave mode" actually is. Is it what's more commonly referred to as "non-voting members" or "listeners" in the literature?