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.
it's the approximate size of the key range in file-system level compressed bytes, specified in the Engine interface comment. Can't hurt to mention it again.