[buteo-sync-plugin-caldav] Don't report missing etags as remote deletion when out of span, contributes to MER#1835.
Edit: completely rewritten MR, using another approach. The only issue is with missing reported etags seen as remote deletions. This can be easily solved by looking if the local incidence intersects with the time span asked to the server. For exceptions and recurring events, this is a bit more tricky and the current MR don't catch properly exceptions outside the window that should be deleted because parent incidence span inside the window and it has been remotely deleted. But it's already better than cleaning the local storage…
In addition, mkcal!2 (closed) is not needed anymore.
Previous attempt: Based on mkcal!2 (closed), this MR will use incidences from the same time window than the server instead of all incidences to do the comparison with the server incidences. This should prevent spurious deletion of events outside the window.
The delta calculation test is updated accordingly (previously we were not checking the full content of the remote deletion list because it did indeed contain additional entries that were not expected to be freed, so now we check that it only contains the two expected incidences).