Add ApproximateSize() method to Engine

Commit adds ApproximateSize() method on the Engine interface, designed to
utilize the GetApproximateSize() method of the rocksdb engine.

+ Implemented in both rocksdb and inMem Engine structs
+ Adds compression-aware test for the method.
Use "start, end Key" instead of keyRange. We don't use keyRange anywhere else in this interface, so keep it consistent. I'm working on a change where I removed engine.KeyRange.