feat: Convert waiting and starting status to 202

[msfstef]

Before we were returning 503s for anything other than active status - for some health check services (like AWS Fargate) you can only rely on the status code, and technically Electric in waiting and active status is ready to accept requests, just not service them.

I've changed it so that:

• waiting and starting return 202, requests are accepted and held but failed after a timeout if not activated
• active returns 200
• stopping returns 503, we haven't implemented it yet anyway

Not sure if we should consider this a breaking change and how we handle releases at the moment @KyleAMathews .

[KyleAMathews]

Seems more like a bug fix than a breaking change so

[robacourt]

👍

[msfstef]

Added a HEALTHCHECK to the dockerfile as well, so that we leverage Docker's "healthy" status too