BUG/MEDIUM: mux-h2: Properly handle shutdowns when received with data

BUG/MEDIUM: mux-h2: Properly handle shutdowns when received with data

The H2_CF_RCVD_SHUT flag is used to report a read0 was encountered. It is
used by the H2 mux to properly handle shutdowns. However, this flag is only
set when no data are received. If it is detected at the socket level when
some data are received, it is not handled. And because the event was
reported on the connection, any other read attempts are blocked. In this
case, we are unable to close the connection and release the mux
immediately. We must wait the mux timeout expires.

This patch should fix the issue #1231. It must be backported as far as 2.0.