stream: remove {writeableState/readableState}.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 readLength and writeLength getter.
cc @nodejs/streams
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows commit guidelines
Affected core subsystem(s)
streams, tls, net