MINOR: stats: protect proxy iteration via watcher

MINOR: stats: protect proxy iteration via watcher

Define a new <px_watch> watcher member in stats applet context. It is
used to register the applet on a proxy when iterating over the proxies
list. <obj1> is automatically updated via the watcher interaction.
Watcher is first initialized prior to stats_dump_proxies() invocation.

This guarantees that stats dump is safe even if applet yields and a
backend is removed in parallel.