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: https://github.com/nodejs/node/pull/29124
PR-URL: https://github.com/nodejs/node/pull/29122


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