render asynchronously

[maxheld83]

rendering the email and other expensive operations should work synchronously so as not to block the session

[maxheld83]

This is now elevated to a bug, because:

1. We currently run in only one instance at a time, and though that can be beefed up, we need to become more efficient and share instances.
2. This keeps getting confused for other bugs shinyapp does not start when a check is running #130 intermittent hard app crash #125 maybe app became unresponsive during test #89
3. Even after removing a bunch of expensive calls (open apc, gone as per remove openAPC #106), we're still darn slow speed up requests #133 so this bites.

Reasons why this was blocked so far, and may still be are listed in #136.