Change to StatsD (#47)

* Use StatsD, not Stackdriver

* go mod tidy

Add a -nobars option to quiet log output

Provide a higher-performance health check (#42) (#45)

This changes the health check from #41 from a database-check to a local
timestamp updated by all the CT downloader threads.

Fix #40: Add a health check (#41)

* Add the infrastructure to do health checks

* Use the update time instead of entry time for the health check

* Use a configuration value and more explicit error reporting

* Wire up the server address configuration to the health server

Add a config entry for Stackdriver

Add the NoopBackend to the engine config as the fallback if disk and …

…firestore aren't configured

More details for exiting with improper usage

add a bunch more metrics

Enable Gomodules

Update to mpb 3.3.4 using contexts