Move level0 sorting logic from Version::SaveTo() to Version::Finalize()

Summary: I realized that "D14409 Avoid sorting in Version::Get() by presorting them in VersionSet::Builder::SaveTo()" is not done in an optimized place. SaveTo() is usually inside mutex. Move it to Finalize(), which is called out of mutex.

Test Plan: make all check

Reviewers: dhruba, haobo, kailiu

Reviewed By: dhruba

CC: igor, leveldb

Differential Revision: https://reviews.facebook.net/D14607

db/version_set.cc

@@ -1110,12 +1110,6 @@ class VersionSet::Builder {
         MaybeAddFile(v, level, *base_iter);
       }
     }
-    // Pre-sort level0 for Get()
-    if (vset_->options_->compaction_style == kCompactionStyleUniversal) {
-      std::sort(v->files_[0].begin(), v->files_[0].end(), NewestFirstBySeqNo);
-    } else {
-      std::sort(v->files_[0].begin(), v->files_[0].end(), NewestFirst);
-    }
 
     CheckConsistency(v);
   }
@@ -1681,6 +1675,12 @@ void VersionSet::MarkFileNumberUsed(uint64_t number) {
 
 void VersionSet::Finalize(Version* v,
   std::vector<uint64_t>& size_being_compacted) {
+  // Pre-sort level0 for Get()
+  if (options_->compaction_style == kCompactionStyleUniversal) {
+    std::sort(v->files_[0].begin(), v->files_[0].end(), NewestFirstBySeqNo);
+  } else {
+    std::sort(v->files_[0].begin(), v->files_[0].end(), NewestFirst);
+  }
 
   double max_score = 0;
   int max_score_level = 0;