Health endpoint when service ready

[conorsch]

Syncing takes a long time: currently ~15m for 200k blocks, or one week of a testnet. This time to sync will grow. Currently, restarting Galileo for any reason, such as deploying a code fix (#72), causes ~15m of downtime while the service syncs.

If Galileo had a /healthz endpoint, we could return non-200 while view server is syncing, and 200 OK when it's ready. Doing that would allow us to roll over Galileo deployments with minimal downtime.

However, at root we want syncing to be fast.