Support throttling/retries to avoid Sendgrid API rate limits.

[mrfelton]

Issue Summary

Sendgrid has rate limits on all of its web API's. These limits can be easy to hit if you are processing multiple items in some kind of loop.

This package should support these rate limits and enable automatic queueing of requests once the limits have been reached, and automatic retries of failed messages.

[thinkingserious]

Great idea @mrfelton!

We have added this to our backlog. For it to move up our queue faster, we need +1s, comments or a pull request with a signed CLA.

[thinkingserious]

Thanks for the upvote @albert-the-creator!

[mrfelton]

Initial concept added in #278