When a room has the partial state flag, we may not have an accurate
`m.room.member` event for event senders in the room's current state, and
so cannot perform soft fail checks correctly. Skip the soft fail check
entirely in this case.

As an alternative, we could block until we have full state, but that
would prevent us from receiving incoming events over federation, which
is undesirable.

Signed-off-by: Sean Quah <[email protected]>

Previously, `partial_state=False` either meant "full state" or
"calculate the flag yourself", depending on whether
`state_ids_before_event` was provided. Split out the latter meaning into
`partial_state=None`.

Signed-off-by: Sean Quah <[email protected]>

Signed-off-by: Sean Quah <[email protected]>

…t can return partial state

Signed-off-by: Sean Quah <[email protected]>

…rwise we can block when receiving or backfilling events over federation

Signed-off-by: Sean Quah <[email protected]>

…e race

Now that we persist events with partial state, we have to handle
`PartialStateConflictError`s, by retrying just once.

Signed-off-by: Sean Quah <[email protected]>

Co-authored-by: Richard van der Hoff <[email protected]>