1. Migrate threads to DB when needed
2. Use threads from DB and avoid fetching from keyserver when data is present.

Depends on D9956

Test plan was done twice, in Chrome and Safari:

1. Check if migrating works
2. Check if threads are not fetched from keyserver when there are in DB
3. Modify thread and reload app
4. Modify thread on different account
5. Close app, modify thread on different account and open app (test if checkpointing works)

I tested this on the account with ~4k threads, I didn't notice any perf downgrade, moreover, I think initial app load was faster when reading threads from DB.