stream: add flow and buffer properties to streams

This adds computed properties to readable and writable streams to
allow access to the readable buffer, the writable buffer, and flow
state without accessing the readable or writable state.

These are the only uses of readable and writable state in the docs
so adding these work arounds allows them to be removed from the docs.

This also updates net, http_client and http_server to use the new
methods instead of manipulating readable and writable state directly.

See: https://github.com/nodejs/node/issues/445
PR-URL: https://github.com/nodejs/node/pull/12855


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>