Support Index generation on the fly during shard acquisition

Lotus Miners will delete the dagstore repo if they have to rollback for some reason. This means that retrieval deals will fail for the (storage deal with upgraded miner -> miner rolls-back and deletes the dagstore repo -> miner upgrades and we do a retrieval) scenario because AcquireShard will fail to find an Index. I think we should allow index generation on the fly during shard acquisition for scenarios such as these where the underlying Mount returns valid data but the Index has gone missing. We can have an option to disable this if needed.

I think a lot of clients will face scenarios such as these.