MINOR: mux-h2: always consider a server's max-reuse parameter

MINOR: mux-h2: always consider a server's max-reuse parameter

This parameter allows to limit the number of successive requests sent
on a connection. Let's compare it to the number of streams already sent
on the connection to decide if the connection may still appear in the
idle list or not. This may be used to help certain servers work around
resource leaks, and also helps dealing with the issue of the GOAWAY in
flight which requires to set a usage limit on the client to be reliable.

This must be backported to 1.9.