refactor timeslot API filters

This MR:

• removes default values for start and end filter (see #189 (closed))
• renames start to startsAfter and end to endsBefore
• adds datetime support to startsAfter and endsBefore
• adds new datetime filter startsBefore and endsAfter

approved this merge request

mentioned in commit 2bf226d5

merged

Thanks for your contribution!

mentioned in issue #189 (closed)