Refactor `get_events_as_list` (#5699)

A couple of changes here:

* get rid of a redundant `allow_rejected` condition - we should already have filtered out any rejected
  events before we get to that point in the code, and the redundancy is confusing. Instead, let's stick in
  an assertion just to make double-sure we aren't leaking rejected events by mistake.

* factor out a `_get_events_from_cache_or_db` method, which is going to be important for a 
  forthcoming fix to redactions.