Refactor read path

As part of the pluggable storage engine effort, we'd like to modularize the read path related code, make it to be independent from existing storage engine implementation details.

For now, refer to https://docs.google.com/document/d/1suZlvhzgB6NIyBNpM9nxoHxz_Ri7qAm-UEO8v8AIFsc for high level designs.