[RFC] Adds Ping() to client/scheduler/server

[pbarnum]

No description provided.

[pbarnum]

@hibiken, this should close #445

[mdaliyan]

Right now I'm pushing a task with an empty payload and then I'm ignoring it when I consume it for the sake of health check. Exposing Ping makes it much easier and cleaner health check than my approach. Please consider increasing the priority for this PR if possible. Thank you.

[korourke9]

This would be a huge help in re-establishing connections automatically rather than manually when Redis gets a new IP. Please consider giving this some attention @hibiken

[bojanz]

@hibiken What can we do to push this along? I need a client.Ping() to be able to integrate into my app's health check metrics.

[arpitflex]

We are deploying an asynq queue alongside our services in a Kubernetes cluster, and a readiness check like this would be very helpful to ensure better Pod restarts by the orchestrator. Are we looking to include this with the 0.25 release @hibiken?