-spec set_member(classy:cluster_id(),
classy:site(), classy:site(),
boolean()) -> {ok, clock()} | {error, _}.
Low-level call that sets Target’s membership state to true.
WARNING: this function does not check if target site exists and/or is part of cluster.
When called with invalid Target,
it will create a new entry that will eventually make its way to the entire cluster.
This fictitious site will become a member until kicked,
and even then some records about it may be kept around.