http2: allow fully synchronous `_final()`
/cc @mcollina
HTTP/2 streams do not use the fact that the native
StreamBase::Shutdown()
is asynchronous by default and
always finish synchronously.
Adding a status code for this scenario allows skipping an
expensive MakeCallback()
C++/JS boundary crossing.
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
commit message follows commit guidelines