BUG/MEDIUM: backend: never try to attach to a mux having no more stream available

BUG/MEDIUM: backend: never try to attach to a mux having no more stream available

The code dealing with idle connections used to check the number of streams
available on the connection only to unlink the connection from the idle
list. But this still resulted in too many streams reusing the same connection
when they were already attached to it.

We must detect that there is no more room and refrain from using this
connection at all, and instead fall back to the no-reuse case. Ideally
we should try to search among other idle connections, but for a backport
let's stay safe.

This must be backported to 1.9.