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>

test/parallel/test-https-truncate.js

@@ -57,7 +57,7 @@ function httpsTest() {
 
 const test = common.mustCall(function(res) {
   res.on('end', common.mustCall(function() {
-    assert.strictEqual(res._readableState.length, 0);
+    assert.strictEqual(res.readableLength, 0);
     assert.strictEqual(bytes, data.length);
   }));