http2: allocate on every chunk send

Previously, we were using a shared stack allocated buffer to hold
the serialized outbound data but that runs into issues if the
outgoing stream does not write or copy immediately. Instead,
allocate a buffer each time. Slight additional overhead here,
but necessary.

Later on, once we've analyzed this more, we might be able to
switch to a stack allocated ring or slab buffer but that's a
bit more complicated than what we strictly need right now.

PR-URL: https://github.com/nodejs/node/pull/16669


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Khaidi Chu <i@2333.moe>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>