R1:2373185e9b62

Conpherence - introduce ConpherenceThreadManager

Summary:
Ref T7014. Fixes T7473. This adds a class to handle thread state about what thread is loaded and what transaction we've seen last. It is deployed 100% in the durable column and only partially deployed in the regular view. Future diff(s) should clean up regular view. Note ConpherenceThreadManager API might change a bit at that time.

Also includes a bonus bug fix so logged out users can't toggle this column

Test Plan: tried to use durable column…