BUG/MEDIUM: promex: server iteration may rely on stale server

BUG/MEDIUM: promex: server iteration may rely on stale server

When performing a promex dump, even though we hold reference on server
during resumption after a yield (ie: buffer full), the refcount mechanism
only guarantees that the server pointer will be valid upon resumption, not
that its content will be consistent. As such, sv->next may be garbage upon
resumption. Instead, we must rely on the watcher mechanism to iterate over
server list when resumption is involved like we already do for stats and
lua handlers.

It must be backported anywhere 071ae8ce3 (" BUG/MEDIUM: stats/server: use
watcher to track server during stats dump") was (up to 2.8 it seems)