Use DestroyColumnFamilyHandle instead of directly deleting column family handle (#6505)

Summary:
Update example usage of closing column family.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6505

Test Plan: cd examples && make column_families_example && ./column_families_example

Differential Revision: D20362100

Pulled By: cheng-chang

fbshipit-source-id: 493c5e0068a40b4f237f8f8511cddd22dc15ea5c

examples/column_families_example.cc

@@ -28,7 +28,8 @@ int main() {
   assert(s.ok());
 
   // close DB
-  delete cf;
+  s = db->DestroyColumnFamilyHandle(cf);
+  assert(s.ok());
   delete db;
 
   // open DB with two column families
@@ -64,7 +65,8 @@ int main() {
 
   // close db
   for (auto handle : handles) {
-    delete handle;
+    s = db->DestroyColumnFamilyHandle(handle);
+    assert(s.ok());
   }
   delete db;