ColumnFamilySet

Summary:
I created a separate class ColumnFamilySet to keep track of column families. Before we did this in VersionSet and I believe this approach is cleaner.

Let me know if you have any comments. I will commit tomorrow.

Test Plan: make check

Reviewers: dhruba, haobo, kailiu, sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15357

include/rocksdb/db.h

@@ -29,6 +29,8 @@ extern const ColumnFamilyHandle default_column_family;
 struct ColumnFamilyDescriptor {
   std::string name;
   ColumnFamilyOptions options;
+  ColumnFamilyDescriptor()
+      : name(default_column_family_name), options(ColumnFamilyOptions()) {}
   ColumnFamilyDescriptor(const std::string& name,
                          const ColumnFamilyOptions& options)
       : name(name), options(options) {}