Contacts lookup fix

Summary

Refactor contacts-getting code to be either "everything" or "one"

Fixes bug from mailing list conversation dated 12 October 2019

Test Plan

Nothing should be visibly different (except for Aleix, since I guess it wasn't working for him before).

Tested by deleting all contacts and re-synchronizing, plus some debugger breakpoints to make sure things looked okay.

Before:

If there were too many contacts, we would get a sqlite crash from trying to look all of them up from a too-long "IN" clause.

After:

We no longer specifically select every contact, so it should work no matter how many there are.