We accept stupid events over backfill, causing state res storms

[richvdh]

If somebody creates an event with hundreds of prev_events, we will (since #3118) reject it when it arrives over federation. However, we may then try to backfill it, and will accept it and do an awful state resolution.

Fixing this is tricky as simply rejecting the event would break back-pagination for the room in question, in the same way that it is currently broken by events which don't have the correct signature (#3121). A solution to #3121 would probably mean that we could also fix this.

[richvdh]

now that #3121 is fixed, it may be more tractable to fix this one. Throwing it into the "validation" bucket.