Reserve runners for the whole duration of their use

[MrSerth]

This commit changes how runners are reserved and released. Previously, a runner was reserved only for the actual time it was actively used and the aim was to reserve it for a time as short as possible. The previous mechanism worked, but had some drawbacks:

For example, a learner could start a submission scoring, which would perform (1) a file copy, (2) an execution of the first test file, (3) an execution of the second test file, etc. For each of these actions, the runner would be reserved and released. Now, it could happen that another run was triggered between the execution of both test files, causing issues with the remaining scoring.

In the refactored version, a runner is reserved once before initially copying files and only released after the last command finished. This ensures that no one can interrupt an execution started once, hopefully making it more robust (and faster, since less locks are required).

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 9 lines in your changes missing coverage. Please review.

Project coverage is 69.60%. Comparing base (f289500) to head (05ab2d7).
Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
app/models/submission.rb	50.00%	7 Missing ⚠️
app/models/runner.rb	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2594      +/-   ##
==========================================
+ Coverage   69.56%   69.60%   +0.04%     
==========================================
  Files         202      202              
  Lines        6370     6376       +6     
==========================================
+ Hits         4431     4438       +7     
+ Misses       1939     1938       -1     

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