Fix sync bug when accepting invites

[richvdh]

Hopefully this will fix #4422: essentially we need to make sure that the
cache on get_rooms_for_user_with_stream_ordering is invalidated before the
SyncHandler is notified for the new events. Doing so on the events stream
rather than the caches stream achieves this, and should be safe because the
only time we need to invalidate this cache is when we persist an event.

[codecov]

Codecov Report

Merging #4903 into develop will decrease coverage by 23.97%.
The diff coverage is 25%.

@@             Coverage Diff              @@
##           develop    #4903       +/-   ##
============================================
- Coverage    77.96%   53.98%   -23.98%     
============================================
  Files          326      326               
  Lines        33958    33959        +1     
  Branches      5601     5601               
============================================
- Hits         26474    18334     -8140     
- Misses        5863    14400     +8537     
+ Partials      1621     1225      -396

[richvdh]

Sadly this approach turns out to be flawed. In general we cannot rely on using the event stream to invalidate caches driven by the current_state_events table (of which get_rooms_for_user_with_stream_ordering is an example), because state resolution means that current_state_events can change in a way that is apparently unrelated to the event which triggers it.

I will investigate whether we can use the "current state delta" replication stream in some way.