BUG/MEDIUM: proxy: protect show backend against be deletion

BUG/MEDIUM: proxy: protect show backend against be deletion

Command "show backend" loops over the list of visible proxies to display
information about all the backend instances. This command may yield in
case of a long output, but this is not safe with introduction of dynamic
backend deletion at runtime.

Fixes this by using the watcher mechanism, similarly to what is
implemented for stats dump. To support this, a new context dedicated to
"show backend" has been defined.

This must be backported up to 3.4.