Only load calendar once

Summary:
When creating an ETMCalendar from an existing etm and setting some
options it would happen that the data is loaded and removed multiple
times because internal model resets (which made the whole process very
expensive).
If we instead delay loading a little and make sure it only happens
once, by giving the caller some time to set options, we can avoid all
of that overhead.

Phabricator: git.kolab.org/T1242

Reviewers: knauss

Reviewed By: knauss

Differential Revision: https://git.kolab.org/D158