Use correct URI for DELETE requests.

We cannot use incidence.customProperty("buteo", "uri") since the storage backend removes all custom properties from deleted incidences. We also cannot use its UID as there is, in general, no correlation between the UID of an incidence and its URI on the server.

However, we only DELETE an incidence if it is still on the server, and all locally deleted incidences which are still on the server are downloaded by fetchRemoteChanges. Therefore we can determine the URI by searching mReceivedCalendarResources for the deleted incidence.
Is mReceivedCalendarResources populated with all existing incidences (from libsocialcache?) prior to detecting local delta / deletion from mkcal?