Programming exercises: Speedup archival

[krusche]

Archiving programming exercises as part of a course or exam was done sequentially which can take quite some time, especially in courses with between 10.000 and 80.000 repositories (such as on the TUM production system).

This PR speeds up the process by running the clone and zip operation in 10 parallel threads. In a local test, the archival time of one course was around 5-7x faster with this approach.

Checklist

General

• I tested all changes and their related features with all corresponding user types on a test server.
• I chose a title conforming to the naming conventions for pull requests.

Server

• Important: I implemented the changes with a very good performance and prevented too many (unnecessary) database calls.
• I followed the coding and design guidelines.
• I documented the Java code using JavaDoc style.

Steps for Testing

Prerequisites:

• Archive a course (and an exam) with passed end date on the course management detail page
• Download the archive and verify all student repositories are included

Code Review

• Code Review 1
• Code Review 2

Manual Tests

• Test 1
• Test 2

[Strohgelaender]

code lgtm