src: make StreamBase::GetAsyncWrap pure virtual

PR-URL: https://github.com/nodejs/node/pull/13174


Reviewed-By: Andreas Madsen <amwebdk@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>