OPTIM/MINOR: server: restrict <all_servers> list usage

OPTIM/MINOR: server: restrict <all_servers> list usage

<all_servers> global list usage is pretty limited. It is only useful at
postparsing to call srv_preinit() on each server instances. It is
possible to simplify it in two ways.

First, dynamic servers are not added anymore into it. For them,
srv_preinit() is called directly on the "add server" handler. Then,
servers are now removed from the global list once postparsing is over.
Thus, <all_servers> is now never used during runtime. This allows to
convert it from a mt-list to a simple list.

Note that log servers and haload servers are created after
check_config_validity(). They must be manually removed from
<all_servers>.