support "debounce" for the batch link

[dannycochran]

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

Motivation:

By default, @apollo/client/link/batch-http throttles requests using batchInterval. E.g. the queue of batched requests will fire every batchInterval milliseconds.

It may be desirable to instead debounce those requests so every new inbound request resets the timer. For instance, many mutations which are firing frequently from editing a table.

This PR introduces a simple debounce boolean to the http-batch-link and batch-link.

[brainkim]

🎉

[dannycochran]

@brainkim thanks for the quick review, resolved your comments.

[brainkim]

@benjamn @hwillson lgtm?

[hwillson]

👍 from me - thanks very much @dannycochran!