http2: use getter replace use directly _witableState.finished

add a new getter to duplex stream to replace the property this .writableState.finished of the object that inherited duplex.

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

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines