Use FreeBusy API to generate scheduling time

[MelissaAutumn]

https://developers.google.com/calendar/api/v3/reference/freebusy/query

I somehow missed this api / component for caldav apis and google api. We can use this in-place for generating availability times.

[MelissaAutumn]

Testing this with a couple of calendars and it seems like it significantly speeds up queries (I mean duh). From 3 seconds with 5 calendars of varying degree to about 500ms with sort of the same output. I'll need to dig up what qualifies as "busy" time as we might be counting "busy" different or wrong.