stream: remove usage of *State.highWaterMark

Replaced _readableState.highWaterMark with a .readableHighWaterMark
getter and _writableState.highWaterMark with a .writableHighWaterMark
getter.
The getters are non-enumerable because they break some prototype
manipulation that happen in the ecosystem.

Ref: https://github.com/nodejs/node/issues/445.

PR-URL: https://github.com/nodejs/node/pull/12860


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>