Don't re-fetch thread root if we already have it
The root event of a thread used to arrive with the pagination request,
but this was unspecced and so got changed to simply fetch the root event.
In many (almost all) cases this shouldn't be necessary because the
thread should already have its root event: re-use it if it's already
there. This is only in pagination, so there's no reason to believe that
the root event would have changed and needs to be re-fetched.
This removes a number of duplicate calls to the /event/ endpoint from
the tests.
