Queue for calendar api requests

[gikf]

Discuss your ideas or share your views:

Adding queue for calendar api requests will allow making sure no request, that should be send to the calendar api, is missed.

Despite of best efforts and adding features informing about broken calendar integration, it is inevitable some requests will be missed in the meantime. Putting all requests into queue by default, or once integration gets broken gives ability to keep track of what needs to be tried again.