Add support for limiting based on maximum concurrent connections

[S48-Mo]

As per BigCommerce's API rate-limit spec there can only be 10 concurrent requests to /v3/customers and 3 concurrent requests to all other endpoints. API calls are metered per-store, per-endpoint.
Add an enchancement to honour this limitation.

[joshdifabio]

We should investigate this further; we use a concurrency of >=50 in some places without issue.

[punkstar]

I'd be interested in exploring if we could just monitor any unexpected 429 responses (where expected is defined based on known rate limiting headers) and keep increasing linearly until we get those responses or something more advanced like slow-start.

I'd also be really interested in reporting to the user when rate limiting has been detected to aid debugging.