Decrease GMS join_timeout and increase xPING num_discovery_runs

Our default JGroups configurations have a GMS join_timeout="5000", meaning the first node to start will always wait for 5 seconds before becoming coordinator. It should be safe to use the JGroups default of 2 seconds.

To make it even more safe, we could configure the discovery protocol's num_discovery_runs="3", so that it makes multiple attempts to contact existing nodes during those 2 seconds of waiting. num_discovery_runs is only used for the initial discovery, so there's no danger of creating additional traffic while the node is up.