Move back to the old get_backfill_events and simplify backfill.

We now rely on the marker events to backfill the base insertion event
which puts it as a insertion event extremity. This functionality was
already in place (see `handle_marker_event`) and was an easy transition.
This way, remote federated homeserver will have the insertion extremity
to ask about in backfill and goes down the historical branch no problem
because of the depth order and the rest of the DAG navigation happens as
normal. Yay simplification!

The key breakthrough was discussing all the ways we can find connected insertion events.
https://docs.google.com/document/d/1KCEmpnGr4J-I8EeaVQ8QJZKBDu53ViI7V62y5BzfXr0/edit#bookmark=id.1hbt9acs963h

The three options we came up were:

 - Find by insertion event prev_events (this is what we were doing before)
 - Find connected insertion events by depth
 - Find connected insertion events by the marker event
    - This made the most sense since we already backfill the insertion event
      when a marker event is processed (see `handle_marker_event`).
    - Gets rid of the extra insertion event lookup in backfill because we
      know it's already backfilled from the marker processing.
    - And gets rid of the extra federated lookup we added in this to PR
      to ask whether the homeserver requesting backfill already has the
      insertion event (deciding whether we fork
      to the history branch before we go down the "live" DAG)

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