health-check/ping

Using a similar solution for the callback system (see issue #110) implement a "ping" request to a remote destination so that when an instance fails it can be removed from rotation.

Configurable options should look like:
- `ping_frequency` (in seconds). Note it is critical to allow enough time in between checks on the receiving end to avoid being marked as down.
- `ping_url`: the remote host and endpoint that receives the request
- `ping_retries`: Retry count for failed ping requests (non 200 responses back)

The code should determine what constitutes a working system for its various systems: database, rabbitmq, http)

Start by reading issue #110 to understand the callback system, then locate the health-check and configuration entry points in the repository. Define how ping_frequency, ping_url, and ping_retries apply to remote checks and how database, RabbitMQ, and HTTP systems determine whether a response is working. Done means failed instances can be removed from rotation after the configured retry behavior.

Written by the indexing model from the issue text.