stream: rm {writeable/readable}State.length

As part of the readableState/writableState mega issue #445, this
removes all of the references to .length on those properties and
replaces them with a readableLength and writableLength getter.

See: https://github.com/nodejs/node/issues/445
PR-URL: https://github.com/nodejs/node/pull/12857


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>