Avoid ColumnFamilyDescriptor copy (facebook#10978)

Summary:
Hi. :) Noticed we are copying ColumnFamilyDescriptor here because my process crashed during copy constructor (cause unrelated)

Pull Request resolved: facebook#10978

Reviewed By: cbi42

Differential Revision: D41473924

Pulled By: ajkr

fbshipit-source-id: 58a3473f2d7b24918f79d4b2726c20081c5e95b4

db/db_impl/db_impl_open.cc

@@ -216,7 +216,7 @@ Status ValidateOptionsByTable(
     const DBOptions& db_opts,
     const std::vector<ColumnFamilyDescriptor>& column_families) {
   Status s;
-  for (auto cf : column_families) {
+  for (auto& cf : column_families) {
     s = ValidateOptions(db_opts, cf.options);
     if (!s.ok()) {
       return s;