Mark requests on re-pinned connections to avoid them being pconnPush()ed

Mark requests on re-pinned connections to avoid them being pconnPush()ed
causing "fd_table[conn->fd].halfClosedReader != NULL" comm assertions later.

Forward.cc comments imply that request->flags.pinned is set by ConnStateData
but that is a lie. The flag is set by forward.cc itself. It was set for PINNED
peers having a valid pinned connection only. When we retry a pinned pconn
race, we still have a PINNED peer but the failed connection prevents us from
setting the flag. If we successfuly re-pin later, we must set the flag.

request->flags.pinned essentially means "the connection is or should be
pinned".