Lessen default LeadershipPing interval

20 seconds is a little too long for nodes to wait between elected leader
health checks. Default is now 10 seconds.
Disable quarantining, dial back health sensitivity

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.
De-stringify non-string config values

Fetch these values from config with the appropriate types. They must be
set with these types in the configuration. Indicate the necessary type
in the SiriusConfiguration object documentation.
Adjust values to line up with 2.3 defaults
prune dead members from memberAgents

dead members are pruned from the membershipAgent based on the following
formula:

```
allowedPingFailures * pingInterval + pingInterval / 2
```

This makes dead membership handling more proactive than just checking
the cluster config
add fixes
Introduce .getDouble and .getInt to SiriusConfig

These attempt to read a value as the particular type, and if that fails,
to convert it to such. Not a ton of crazy logic, but since
SiriusConfiguration takes types (not just Strings), it's been kind of
like trying to hit a moving target sometimes to populate your config.
This makes it more flexible.
More precise downcast for .getProp

@smuir pointed out that the downcast inside the Option doesn't do any
checking (beyond checking the Option, of course). It doesn't much
matter here, since it's an Any (which we can't do any checking on,
anyway), but if that ever changed, this could be a weird hole for
bugs to hide in.
Merge pull request #92 from tbarker9comcast/improve-membership-ping

add membership health check
Merge pull request #90 from clinedome-work/unstring-config-values

De-stringify non-string config values
Merge pull request #89 from clinedome-work/loosen-heartbeat-configs

Loosen heartbeat configs