MINOR: stats: use stress mode to force reentrant dumps

MINOR: stats: use stress mode to force reentrant dumps

Provide alternative code during stats dump when stress mode is active.
The objective is to force the applet to yield on every output line. This
allows to easily test reentrant code paths, in particular while adding
and removing server instances.

To support this, output is interrupted every time the output buffer (or
its equivalent) is not empty. Use COND_STRESS() macro to provide default
and stress alternative conditions.