Carefully filter out non matching time ranges for CalDAV search

When we search for CalDAV objects in the DB we take the first and last
occurence into account. For recurring events that is when they take
place the very first time and the very last time. Searching in a more
specific time range will still match this condition, because the
recurring event starts before the end of the requested range but ends
after the start of the requested range.

Sabre has filters for this. If we apply them on all seach objects of a
search with a time range, then only the recurring events actually taking
place at the time of the requested time range will be returned.

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>