message: Use unordered_set for perf win
diff: @@ -1231,12 +1231,12 @@ stripe::StatementIt Scheduler::AddSwapOut(stripe::StatementIt si, CacheEntry* en
  }
  void Scheduler::RebuildTransitiveDeps() {
  - std::unordered_map<stripe::StatementIt, std::set<stripe::StatementIt>> tdeps;
  + std::unordered_map<stripe::StatementIt, std::unordered_set<stripe::StatementIt>> tdeps;
  tdeps.reserve(block_->stmts.size());
  for (auto sit = block_->stmts.begin(); sit != block_->stmts.end(); ++sit) {
  - std::set<stripe::StatementIt> stmt_deps;
  - std::set<stripe::StatementIt> stmt_tdeps;
  + std::unordered_set<stripe::StatementIt> stmt_deps;
  + std::unordered_set<stripe::StatementIt> stmt_tdeps;
  for (auto dep : (*sit)->deps) {
  stmt_deps.emplace(dep);
  stmt_tdeps.insert(tdeps[dep].begin(), tdeps[dep].end());
  