rework shadownaemon downtime/comment synchronisation

removed the downtime/comment synchronisation based on the last highest id. That
way sometimes lead to situations where the host had still
scheduled_downtime_depth > 0 set but no belonging downtimes exists which broke
nagvis and thruk and probably others too.

Instead we now check all downtime/comment ids all the time and synchronize them one
by one by fetching the required data when its missing. In addition we force updating
the corresponding host and service when removing/adding downtimes/comments.

Signed-off-by: Sven Nierlein <sven@nierlein.de>