Debug missing hosts after siafile upgrade

It looks like fileToSiaFile sometimes fails to resolve the correct host public key from a contract id if said contract has been renewed. The reason for that seems to be that the contracts within legacy files don't get their contract id updated after the contract is renewed. Only when the chunk is repaired for the first time, the new contract is added to the contracts of the legacy file. To fix that, this MR will now also look at old contracts to create a mapping from contract id to public key.