Add slot to the gsfa index + add a way to read the transactions for an address between two slots

[linuskendall]

To support the backfill of transactions that mention specific accounts by slot range, we would need to add a way to read signatures for address between two slots.

This will require adding the slot number to the gsfa index for efficient retrievals.

When streaming transactions out we'd find the start slot number in the gsfa index for the particular acocunt key, and then start reading the index forwards until we find the end slot number.