Support for column families in TTL DB

Summary:
This will enable people using TTL DB to do so with multiple column families. They can also specify different TTLs for each one.

TODO: Implement CreateColumnFamily() in TTL world.

Test Plan: Added a very simple sanity test.

Reviewers: dhruba, haobo, ljin, sdong, yhchiang

Reviewed By: haobo

CC: leveldb, alberts

Differential Revision: https://reviews.facebook.net/D17859

include/rocksdb/write_batch.h

@@ -65,9 +65,7 @@ class WriteBatch {
 
   // If the database contains a mapping for "key", erase it.  Else do nothing.
   void Delete(ColumnFamilyHandle* column_family, const Slice& key);
-  void Delete(const Slice& key) {
-    Delete(nullptr, key);
-  }
+  void Delete(const Slice& key) { Delete(nullptr, key); }
 
   // Append a blob of arbitrary size to the records in this batch. The blob will
   // be stored in the transaction log but not in any other file. In particular,