Loosen heartbeat configs

Quaranting actually seems to be detrimental to us right now. We maintain
our own cluster membership and test for connectivity, so the double-duty
is confusing matters. We're also seeing a bit too much churn with the
health checks, so dialing that back to a more generous level.

These could reasonably go back to defaults (or at least, closer to them)
if we changed the way we handle actor references (using ActorSelections,
perhaps) then we may want to adjust these again.

Also, 20 seconds is a little too long for nodes to wait between elected leader
health checks. Default is now 10 seconds.
Yeah, true. I added the commit message from the 'Lessen interval' commit to the PR message. The two changes here (dial back sensitivity, decrease ping interval) are related (in that they're tweaking the way we verify liveness and membership) but don't directly affect one another.

This change doesn't quite make it less conservative -- the threshold for leader liveness is still the same -- but it does mean that we'll be able to respond to changes a little more quickly.