[pvr.hts] Improve sync after connection restore.

tvheadend may running (on another server) while xbmc is suspended. In case timers/recordings/channels/tags are modified while xbmc is not active and xbmc gets resumed, it will not handle timers/recordings/... deleted meanwhile in tvheadend. This PR fixes this issue.
Shouldn't we just bug out in case that happens? Since you can't do anything anyway with a broken connection to the backend except looking at all the entries and wondering why they all fail when clicking them.