net,http2: refactor _write and _writev

Refactor writable part (the _write and _writev functions) in net.Socket
and http2.Http2Stream classes.
Also involves adding a generic "WriteGeneric" method to the Http2Stream
class based on net.Socket._writeGeneric, but behind a symbol.

Backport-PR-URL: https://github.com/nodejs/node/pull/22850
PR-URL: https://github.com/nodejs/node/pull/20643


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>