op-challenger: Handle proposals beyond current chain heads

[ajsutton]

Challenger has to handle the case where the proposed timestamp is in the future so when it requests a super root from supervisor there is no data because the chains haven't reached that timestamp yet. Need to work out what supervisor will return in that case, detect it in challenger and use the invalid hash as the claim (since it is beyond the safe head).

This is in addition to the case where we can get a super root but it wasn't safe at the game's L1 head (tracked separately).