BUG/MINOR: server: adjust initialization order for dynamic servers

BUG/MINOR: server: adjust initialization order for dynamic servers

It appears that in cli_parse_add_server(), we're calling srv_alloc_lb()
and stats_allocate_proxy_counters_internal() before srv_preinit() which
allocates the thread groups. LB algos can make use of the per_tgrp part
which is initialized by srv_preinit(). Fortunately for now no algo uses
both tgrp and ->server_init() so this explains why this remained
unnoticed to date. Also, extra counters will soon require per_tgrp to
already be initialized. So let's move these between srv_preinit() and
srv_postinit(). It's possible that other parts will have to be moved
in between.

This could be backported to recent versions for the sake of safety but
it looks like the current code cannot tell the difference.