http2: general cleanups in core.js

* fixup js debug messages * simplify and improve rstStream * improve and simplify _read * simplify and improve priority * simplify on ready a bit * simplify and improve respond/push * reduce duplication with _unrefActive * simplify stream close handling PR-URL: #17209 Reviewed-By: Anatoli Papirovski <[email protected]>

Author: jasnell