Instead of key_string::Value, all functions in SortedDataInterface should accept an unowned parameter (preferably SortedDataKeyValueView) that contains the correctly encoded key and value format for the underlying index type.

Goals:

• Each index implementation does not have to perform any decoding or re-encoding to insert or remove from the index. All work is done by the caller and all SDI API functions accept an opaque key that is in exactly the right format for inserting, deleting, or seeking.
• No functions on SortedDataInterface accept a key_string::Value.

Implementation details:

• Each SDI exposes an implementation-defined generator that constructs opaque keys given an allocator, BSON, and a RecordId.
• This generator can generate keys for insertion, deletion, and seeking.