http2: shrink default vector::reserve() allocations

Allocating memory upfront comes with overhead, and in particular,
`std::vector` implementations do not necessarily return memory
to the system when one might expect that (e.g. after shrinking the
vector).

Backport-PR-URL: #29124
PR-URL: #29122
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: addaleax

src/node_http2.cc

@@ -668,7 +668,7 @@ Http2Session::Http2Session(Environment* env,
   // fails.
   CHECK_EQ(fn(&session_, callbacks, this, *opts, *allocator_info), 0);
 
-  outgoing_storage_.reserve(4096);
+  outgoing_storage_.reserve(1024);
   outgoing_buffers_.reserve(32);
 
   {
@@ -1993,9 +1993,10 @@ Http2Stream::Http2Stream(
 
   // Limit the number of header pairs
   max_header_pairs_ = session->GetMaxHeaderPairs();
-  if (max_header_pairs_ == 0)
-  max_header_pairs_ = DEFAULT_MAX_HEADER_LIST_PAIRS;
-  current_headers_.reserve(max_header_pairs_);
+  if (max_header_pairs_ == 0) {
+    max_header_pairs_ = DEFAULT_MAX_HEADER_LIST_PAIRS;
+  }
+  current_headers_.reserve(std::min(max_header_pairs_, 12u));
 
   // Limit the number of header octets
   max_header_length_ =