Check snapshot isolation of signature transactions

[eddyashton]

We may break snapshot isolation because signature transactions are special.

There is a delay between booking a version for a signature, and producing the corresponding write. What happens if a separate transaction tries to read from the signature table in that gap? Does it read an inaccurate value, or do we separate the booked-version and readable-version? Should you be allowed to read from the signature table in other transactions?