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.
Slightly non-DRY. Could merge insertKeys and this one here by inserting values[idx] if defined or []byte("value").