Scheduler rework

• request processing should not depend on DPA queue as there is no interaction with the network
• use UUIDv4 as task ID instead of signed integers
• respect existing task IDs instead of always generating new ones (generate new on collision)
• select execution condition and store only relevant information (currently if-elseif-else)
• accept ISO time format and take timezone into account when processing one-shot task execution
• edit existing task instead of having to remove old task and create new (also leads to new task ID)
• complete jsonschema specification (w/o messages)

Relevant issues:
#168 (closed)
#169 (closed)

@ondracekr @spinarr Input on these would be great, additional suggestions or specs. To be discussed.