http: align parser with StreamBase interface changes

The `StreamBase` interface changed, so that `OnStreamRead()`
and `OnStreamAlloc()` are not guaranteed to be emitted in the
same tick any more.

This means that, while it isn’t causing any trouble right now,
we should not assume that it’s safe to return a static buffer
in the HTTP parser’s `OnStreamAlloc()` method.

PR-URL: https://github.com/nodejs/node/pull/18936


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>