Memtable scan params are too deep coupling with `skiplist` impl

[Rachelint]

Describe This Problem

I found ScanRequest, the main scan param is too deep coupling with skiplist impl, which exposes the low level concepts like xxx_key, sequence. As I see, it hurts the extensibility of the interface deeply...

Proposal

I think somethings like projection and filter exprs should be placed in ScanRequest rather than the too low level concepts only meaning in a specific impl.

Additional Context

No response