R1:559c0fe88663

Tune cookie behaviors for 'phcid', 'phreg', etc

Summary:
Fixes T3471. Specific issues:

- Add the ability to set a temporary cookie (expires when the browser closes).
- We overwrote 'phcid' on every page load. This creates some issues with browser extensions. Instead, only write it if isn't set. To counterbalance this, make it temporary.
- Make the 'next_uri' cookie temporary.
- Make the 'phreg' cookie temporary.
- Fix an issue where deleted cookies would persist after 302 (?) in some cases…