http2: delay closing stream

[apapirovski]

Delay automatically closing the stream with setImmediate in order to allow any pushStreams to be sent first. Also disable the readable side of the pushStream since there should never be any data incoming.

I'm not able to make a test since technically what we were doing was fine (and works with our own client) but the browsers seem a bit too strict.

Fixes: #20992

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines