A path for disabling sync XHR

Synchronous XHR is bad for performance, and doing blocking network access on your UI thread is a long-known anti-pattern.  Some thoughts / notes from TPAC discussion [here](https://docs.google.com/document/d/1G4-oIXQBcqysk4PNHBzCE4XAaGURipPTbajfx5iDIRc/edit?usp=sharing)