Make _get_state_map_for_room not break when room state events don't contain an event id.

[Yoric]

When adding a on_new_event handler on a federated homeserver, I keep hitting:

Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/synapse/replication/tcp/handler.py", line 285, in _unsafe_process_queue
    await self._process_command(cmd, conn, stream_name)
  File "/usr/local/lib/python3.9/site-packages/synapse/replication/tcp/handler.py", line 300, in _process_command
    await self._process_rdata(stream_name, conn, cmd)
  File "/usr/local/lib/python3.9/site-packages/synapse/replication/tcp/handler.py", line 516, in _process_rdata
    await self.on_rdata(stream_name, cmd.instance_name, cmd.token, rows)
  File "/usr/local/lib/python3.9/site-packages/synapse/replication/tcp/handler.py", line 531, in on_rdata
    await self._replication_data_handler.on_rdata(
  File "/usr/local/lib/python3.9/site-packages/synapse/replication/tcp/client.py", line 212, in on_rdata
    await self.notifier.on_new_room_event_args(
  File "/usr/local/lib/python3.9/site-packages/synapse/notifier.py", line 338, in on_new_room_event_args
    await self._third_party_rules.on_new_event(event_id)
  File "/usr/local/lib/python3.9/site-packages/synapse/events/third_party_rules.py", line 399, in on_new_event
    state_events = await self._get_state_map_for_room(event.room_id)
  File "/usr/local/lib/python3.9/site-packages/synapse/events/third_party_rules.py", line 471, in _get_state_map_for_room
    state_events[key] = room_state_events[event_id]
KeyError: (some event id)

Apparently, we have some invalid state in the database of this homeserver (the event was already in the logs last May) and any attempt to call _get_state_map_for_room will break. The fact that _get_state_map_for_room is solely due to the fact that there was no on_new_event callback on this homeserver until now.

This PR uses the more robust get_current_state to implement _get_state_map_for_room, to avoid such breakages.

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[erikjohnston]

FTR I think this is because a rejected event got into the state, a bit like #6867. We correctly handle this case in other places, so the fix here is just to reuse that code rather than implementing it again separately.

[clokep]

@Yoric please update the title / description of this to match the current fix! Thank you! 😄