BUG/MEDIUM: server: initialize the idle conns list after parsing the config

BUG/MEDIUM: server: initialize the idle conns list after parsing the config

The idle conns lists are sized according to the number of threads. As such
they cannot be initialized during the parsing since nbthread can be set
later, as revealed by this simple config which randomly crashes when used.
Let's do this at the end instead.

    listen proxy
        bind :4445
        mode http
        timeout client 10s
        timeout server 10s
        timeout connect 10s
        http-reuse always
        server s1 127.0.0.1:8000

    global
        nbthread 8

This fix must be backported to 1.9 and 1.8.