By using ChatThreadItemLoaderCache, we avoid overwhelming Hermes with Promises (context here).

Depends on D14140

I tested this task by playing around with the ChatThreadList on mobile while using a stopwatch to measure how long various operations took. I tested the updated code 3 times against both the most recent Testflight build as well as master. I found that performance was approximately the same as before. I tested scrolling down, selecting threads that were read, selecting threads that were unread, and search.