calendar: More fixes with exceptions when not repeating

The fixes from nexedi/erp5!630
were not enough, exceptions set on days were calendar did not repeat
were also confusing other exceptions after this.

Simplify implementation a lot, instead of keeping track of the next
exception date, start by building a set of all exceptions dates and
use membership of this set as a criterion to skip exceptions.

/reviewed-on nexedi/erp5!1030