Allow the bulk processor to slice processing and wait between slices

[hugocorbucci]

When you're sending thousand of emails, you might hit a quota limit like Dreamhost's 100 emails in 1 hour quota or AWS's SES sandbox limit of 200 emails per 24 hours and 1 per second or real limit of 10,000 emails per 24 hours and 5 per second.
If the bulk processor could receive that information, it could sleep between emails as well as pause upon reaching the limit until that limit has cleared.