OPTIM/MEDIUM: proxy/server: avoid server list reordering on startup

Prior to this patch, each server parsed from the configuration was added
at the front of the proxy list. The list was then reversed once parsing
is finished to reflect the configuration order.

Now that proxy servers list has been converted to a doubly linked list,
there is no more a reason for this. Thus, this patch changes the server
insertion order on configuration parsing : this is now performed
directly at the end of the proxy list. Reversal is unnecessary and has
been removed, so post-config performance may be slightly improved.

Peers parsing is the only module which relies on the order insertion.
Thus it has been adapted to now use the last server in its proxy.