Flush sinks on shutdown

[csolidum]

Howdy,
It looks like sinks aren't always being flushed when veneur is being shut down. We're using veneur to handle metrics for jobs running in our kuberenetes cluster. If a job completes and calls the /quitquitquit endpoint before the first flush, we notice that metrics don't always make it to our metrics provider. The expected behavior would be that data is properly flushed when veneur is gracefully shutting down.