Resolve "REST API to query events"

Closes #10 (closed)

Introduce end-points GET /execution/events to get execution events. Filter parameters can be provided as json payload or as query parameters. The result is a json schema {"jobs": [...]}. The list is a list of lists, where events are grouped by job_id.

Example 1: GET /execution/events ?starttime=...&endtime=...&context=job

Result 1:

[[{"job_id": "1", "type"="start", ...}, {"job_id": "1", "type"="end", ...}],
 [{"job_id": "2", "type"="start", ...}, {"job_id": "2", "type"="end", ...}],
 [{"job_id": "3", "type"="start", ...}]]

Example 2: GET /execution/events ?job_id="2"

Result 2:

[
 [{"job_id": "2", "type"="start", ...}, {"job_id": "2", ...}, ... {"job_id": "2", "type"="end", ...}],
]