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.  An underscored method is also added to allow
internal function to set the flow 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.

Part of the readableState/writableState mega issue  nodejs#445.

This changes the docs to use the new readableBuffer, writableBuffer, and flow
properties instead of manipulating the readable and writable state.

Part of the readableState/writableState mega issue nodejs#445.

This updates http_client and http_server to use the new _setFlowing method to
set the flowing state instead of directly manipulating readable state.

Part of the readableState/writableState mega issue nodejs#445.

This uses the new internal method _setFlowing to avoid a manipulation
of readable state and uses writableBuffer instead of the getBuffer method of the
writable state.

Part of the readableState/writableState mega issue  nodejs#445