MINOR: applet: define applet_putchk_stress() alternative

MINOR: applet: define applet_putchk_stress() alternative

Previous patch introduced stress mode to be able to easily test
alternative code paths.

The first point would be to force interruption of stats dump on every
line and check reentrant patchs, in particular while adding and removing
servers instances.

The purpose of this patch is to be able to use applet_putchk_stress()
during stats dump while not impacting other applets. To support this,
extract applet_putchk() into an internal _applet_putchk() which have a
new argument stress. Define two helpers applet_putchk() and
applet_putchk_stress(), the latter to set the stress argument to true.

For the moment, applet_putchk_stress() is not used. This will be the
subject of the next patch.