Add GetID to ColumnFamilyHandle

Summary:
Expose GetID to ColumnFamilyHandle interface so that we can save column
family data by id instead of name.

Test Plan: Testing in MySQL on Rocks.

Reviewers: sdong, igor

Reviewed By: igor

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D32943

include/rocksdb/db.h

@@ -44,6 +44,7 @@ class ColumnFamilyHandle {
  public:
   virtual ~ColumnFamilyHandle() {}
   virtual const std::string& GetName() const = 0;
+  virtual uint32_t GetID() const = 0;
 };
 extern const std::string kDefaultColumnFamilyName;