"multi" Prefix Extractor / SliceTransform

[Stu Hood]

Hey folks! Thanks for all your hard work on RocksDB: we love it.

Our system uses deeply compound keys, a few of which we'd like to be able to scan in. In short: some of the compound key components represent collections: we'd like to be able to iterate in collections.

But we'd like to enable nested collections. And doing that efficiently would seem to require a "multi" `prefix_extractor` / SliceTransform facility, which would allow a single key to become multiple entries in the bloom filter.

Have there been previous requests for this facility? Might a patch be welcome to implement it, if it could be made backwards compatible?

Thanks again!

Stu