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).

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

src/node_http2.cc

@@ -639,7 +639,7 @@ 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);

  {
@@ -1914,7 +1914,7 @@ Http2Stream::Http2Stream(Http2Session* session,
  if (max_header_pairs_ == 0) {
    max_header_pairs_ = DEFAULT_MAX_HEADER_LIST_PAIRS;
  }
  current_headers_.reserve(max_header_pairs_);
  current_headers_.reserve(std::min(max_header_pairs_, 12u));

  // Limit the number of header octets
  max_header_length_ =