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.
Merge pull request #96 from cockroachdb/mtracy/approximate_size

Add ApproximateSize() method to Engine