stream: use plain objects for write/corked reqs

This is similar to a change made awhile back for storing
process.nextTick() requests.

PR-URL: https://github.com/nodejs/node/pull/10558


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