Lower memory usage for aptly db cleanup

[smira]

Description of the Change

This is not a complete fix, but the easiest first step.

During db cleanup, aptly is loading every repo/mirror/... into memory,
and even though each object is processed only once, collection holds
a reference to all the loaded objects, so they won't be GC'd until
process exits.

CollectionFactory.Flush() releases pointers to collection objects,
making objects egligble for GC.

This is not a complete fix, as during iteration we could have tried
to release a link to every object being GCed and that would have
helped much more.

See #761

Checklist

• unit-test added (if change is algorithm)
• functional test added/updated (if change is functional)
• man page updated (if applicable)
• bash completion updated (if applicable)
• documentation updated
• author name in AUTHORS

[codecov]

Codecov Report

Merging #762 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #762   +/-   ##
======================================
  Coverage    63.7%   63.7%           
======================================
  Files          50      50           
  Lines        6271    6271           
======================================
  Hits         3995    3995           
  Misses       1788    1788           
  Partials      488     488

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[sliverc]

Change LGTM.

I guess it might be a good idea whether this could be tested in a real life scenario though before this is merged or maybe released whether it really improves the situation.

[smira]

I will merge this PR and I will start working on a more complete solution at the same time.