default implementation for InRange

Summary:
it's confusing to implementors of prefix extractor to implement an unused function
Closes https://github.com/facebook/rocksdb/pull/2460

Differential Revision: D5267408

Pulled By: ajkr

fbshipit-source-id: 2f1fe3131efc978f6098ae7a80e52bc7a0b13571

include/rocksdb/slice_transform.h

@@ -58,7 +58,7 @@ class SliceTransform {
   virtual bool InDomain(const Slice& key) const = 0;
 
   // This is currently not used and remains here for backward compatibility.
-  virtual bool InRange(const Slice& dst) const = 0;
+  virtual bool InRange(const Slice& dst) const { return false; }
 
   // Transform(s)=Transform(`prefix`) for any s with `prefix` as a prefix.
   //