collectd: adjust reaction to ntp time at boot time

Adjust the reaction to a polling interval timestamp that references
to a past time.

Past timestamps can happen when ntpd adjusts router's time after network
connectivity is obtained after boot. Collectd shows warnings for each plugin
as it tries to enter new values with the same timestamp as the previous one.

This patch adjusts the next polling time to be now+2 seconds for the main
loop and for the plugin-specific read loops. That avoids the warnings, but
does not overreact in case there are shorter polling intervals or the time
gets adjusted for other reasons.

Additionally some debug statements are aded, but they are visible only
when --enable-debug configure option is used in Makefile.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>
(cherry-picked for 19.07)
Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>