Assert keys/values pinned by range deletion meta-block iterators

Summary:
`RangeDelAggregator` holds the pointers returned by `BlockIter::key()` and `BlockIter::value()` so requires the data to which they point is pinned. `BlockIter::key()` points into block memory and is guaranteed to be pinned if and only if prefix encoding is disabled (or, equivalently, restart interval is set to one). I think `BlockIter::value()` is always pinned. Added an assert for these and removed the wrong TODO about increasing restart interval, which would enable key prefix encoding and break the assertion.
Closes https://github.com/facebook/rocksdb/pull/3875

Differential Revision: D8063667

Pulled By: ajkr

fbshipit-source-id: 60b5ebcc0cdd610dd6aad9e74a23378793672c41

db/range_del_aggregator.cc

@@ -195,6 +195,10 @@ Status RangeDelAggregator::AddTombstones(
   input->SeekToFirst();
   bool first_iter = true;
   while (input->Valid()) {
+    // The tombstone map holds slices into the iterator's memory. This assert
+    // ensures pinning the iterator also pins the keys/values.
+    assert(input->IsKeyPinned() && input->IsValuePinned());
+
     if (first_iter) {
       if (rep_ == nullptr) {
         InitRep({upper_bound_});

table/block_based_table_builder.cc

@@ -300,7 +300,7 @@ struct BlockBasedTableBuilder::Rep {
                       : 0),
         data_block(table_options.block_restart_interval,
                    table_options.use_delta_encoding),
-        range_del_block(1),  // TODO(andrewkr): restart_interval unnecessary
+        range_del_block(1 /* block_restart_interval */),
         internal_prefix_transform(_ioptions.prefix_extractor),
         compression_type(_compression_type),
         compression_opts(_compression_opts),

util/testutil.h

@@ -179,6 +179,9 @@ class VectorIterator : public InternalIterator {
 
   virtual Status status() const override { return Status::OK(); }
 
+  virtual bool IsKeyPinned() const override { return true; }
+  virtual bool IsValuePinned() const override { return true; }
+
  private:
   std::vector<std::string> keys_;
   std::vector<std::string> values_;