BUG/MEDIUM: proxy: protect "show servers ..." against server deletion

BUG/MEDIUM: proxy: protect "show servers ..." against server deletion

Command show servers conn/state loops over a server list to display
various information. This command may yield in case of a large output.
This can cause a crash if the server on which the command was paused is
deleted prior to restarting the command.

Fixes this by using the watcher mechanism, similar to what is already
implementing with stats dump.

This must be backported at least up to 3.0. For older releases, watcher
mechanism is not available, server refcount + SRV_F_DELETED flag must be
used instead.