BUG/MEDIUM: h2: enforce the per-connection stream limit

BUG/MEDIUM: h2: enforce the per-connection stream limit

h2spec reports that we unfortunately didn't enforce the per-connection
stream limit that we advertise. It's important to ensure it's never
crossed otherwise it's cheap for a client to create many streams. This
requires the addition of a stream count. The h2c struct could be cleaned
up a bit, just like the h2_detach() function where an "if" block doesn't
make sense anymore since it's always true.

To backport to 1.8.