VersionBuilder: Erase with iterators for better performance

Summary: Closes https://github.com/facebook/rocksdb/pull/3007

Differential Revision: D6077701

Pulled By: yiwu-arbug

fbshipit-source-id: a6fd5b8a23f4feb1660b9ce027f651a7e90352b3

db/version_builder.cc

@@ -274,11 +274,12 @@ class VersionBuilder::Rep {
         auto exising = levels_[level].added_files.find(number);
         if (exising != levels_[level].added_files.end()) {
           UnrefFile(exising->second);
-          levels_[level].added_files.erase(number);
+          levels_[level].added_files.erase(exising);
         }
       } else {
-        if (invalid_levels_[level].count(number) > 0) {
-          invalid_levels_[level].erase(number);
+        auto exising = invalid_levels_[level].find(number);
+        if (exising != invalid_levels_[level].end()) {
+          invalid_levels_[level].erase(exising);
         } else {
           // Deleting an non-existing file on invalid level.
           has_invalid_levels_ = true;