When timezone information changes it is important that we include the
timezone information current at the time when the event was created,
so it is displayed in the right place.

We used to always provide timezone information current at the time of
synchronization (respectively, we cached that information once and never
updated it). This resulted in recurring events that were created before
a timezone info change, to be displayed in the wrong time forever.

With this patch we:

• take dtstart into account when generating the timezone information
• drop the caching of timezone information as it would have to take the date range during which it is valid into account.