* Fix ordering when backfilling
The problem was that we weren't sorting the returned events
by depth when sending them back to the caller, instead we
were sorting by prev_events which is not the same thing.
* Fixup tests
* Correctly generate backpagination tokens for events which have the same depth
With tests. Unfortunately the code around here is hard to understand.
There will be a subsequent PR which fixes this up now that we have a test
harness in place.
* Add postgres impl
* More linting
* Fix psql statement so it actually works
