http2: Abstracting stream and session-related logic
Take session and strems-related logic out of core.js for easier coding (every time I try to make changes to the logic for http2 I see that the file is too big and I can't do anything about it).
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
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