Fixes an issue where XDRs aborted unpredictably
Fixes an issue where XDRs aborted unpredictably

- XDRs are known to abort if too many are sent simultaneously
- This allows sends to execute asynchronously from their setup and space apart more.
- See note on https://developer.mozilla.org/en-US/docs/Web/API/XDomainRequest

Signed-off-by: Christopher Rogers <551f4eee005e3e519560e2feb8988249d292c73e@gmail.com>

recurly/recurly-js,recurly/recurly-js,recurly/recurly-js