Use vector in UncollapsedRangeDelMap (#4487)

Summary:
Using `./range_del_aggregator_bench --use_collapsed=false
--num_range_tombstones=5000 --num_runs=1000`, here are the results before and
after this change:

Before:
```
=========================
Results:
=========================
AddTombstones:           1822.61 us
ShouldDelete (first):    94.5286 us
```

After:
```
=========================
Results:
=========================
AddTombstones:           199.26 us
ShouldDelete (first):    38.9344 us
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4487

Differential Revision: D10347288

Pulled By: abhimadan

fbshipit-source-id: d44efe3a166d583acfdc3ec1199e0892f34dbfb7

db/range_del_aggregator.cc

@@ -37,8 +37,7 @@ struct ParsedInternalKeyComparator {
 // An UncollapsedRangeDelMap is quick to create but slow to answer ShouldDelete
 // queries.
 class UncollapsedRangeDelMap : public RangeDelMap {
-  typedef std::multiset<TruncatedRangeTombstone, TombstoneStartKeyComparator>
-      Rep;
+  typedef std::vector<TruncatedRangeTombstone> Rep;
 
   class Iterator : public RangeDelIterator {
     const Rep& rep_;
@@ -70,7 +69,7 @@ class UncollapsedRangeDelMap : public RangeDelMap {
 
  public:
   explicit UncollapsedRangeDelMap(const InternalKeyComparator* icmp)
-      : rep_(TombstoneStartKeyComparator(icmp)), icmp_(icmp) {}
+      : icmp_(icmp) {}
 
   bool ShouldDelete(const ParsedInternalKey& parsed,
                     RangeDelPositioningMode mode) override {
@@ -78,7 +77,7 @@ class UncollapsedRangeDelMap : public RangeDelMap {
     assert(mode == RangeDelPositioningMode::kFullScan);
     for (const auto& tombstone : rep_) {
       if (icmp_->Compare(parsed, tombstone.start_key_) < 0) {
-        break;
+        continue;
       }
       if (parsed.sequence < tombstone.seq_ &&
           icmp_->Compare(parsed, tombstone.end_key_) < 0) {
@@ -101,7 +100,7 @@ class UncollapsedRangeDelMap : public RangeDelMap {
   }
 
   void AddTombstone(TruncatedRangeTombstone tombstone) override {
-    rep_.emplace(tombstone);
+    rep_.emplace_back(tombstone);
   }
 
   size_t Size() const override { return rep_.size(); }
@@ -109,6 +108,7 @@ class UncollapsedRangeDelMap : public RangeDelMap {
   void InvalidatePosition() override {}  // no-op
 
   std::unique_ptr<RangeDelIterator> NewIterator() override {
+    std::sort(rep_.begin(), rep_.end(), TombstoneStartKeyComparator(icmp_));
     return std::unique_ptr<RangeDelIterator>(new Iterator(this->rep_));
   }
 };