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: https://github.com/nodejs/node/pull/17209


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>