MINOR: proxies: Add a per-thread group field to struct proxy.

MINOR: proxies: Add a per-thread group field to struct proxy.

Add a per-thread group field to struct proxy, that will contain a struct
queue, as well as a new field, "queueslength".
This is currently unused, so should change nothing.
Please note that proxy_init_per_thr() must now be called for each proxy
once the thread groups number is known.