GMS: reduce likelihood of merges on concurrent new member startup

When multiple new members are started concurrently, and no coord is present yet, different clients get different discovery results, and this may lead to merging.

If we have no coord, we could run the discovery protocol multiple time, with the current members list being cached, and then might end up with a more similar list of new members.

This could be governed by an additional attribute in Discovery (num_discovery_runs?)