add a hint so that sourmash gather does not do a prefetch for a particular db?

[ctb]

in #3558, there's not much of a point in doing a prefetch step, since the RocksDB-based RevIndex is quite efficient. Nonetheless, we need to implement the prefetch/CounterGather stuff, which adds complexity, increases memory usage, and reduces efficiency. Maybe let Index classes provide a hint so that prefetch isn't needed/isn't done?