do not keep all annotations in RAM on backup

[Eldies]

Motivation and context

On backup, all the annotations are kept in memory. It can be a problem if the annotations are large.
Fixing it.

How has this been tested?

Checklist

• I submit my changes into the develop branch
• I have created a changelog fragment
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (see GitHub docs)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.98%. Comparing base (7ca3a31) to head (de6d3a1).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #9099      +/-   ##
===========================================
+ Coverage    73.94%   73.98%   +0.03%     
===========================================
  Files          428      428              
  Lines        44532    44585      +53     
  Branches      3881     3890       +9     
===========================================
+ Hits         32930    32987      +57     
+ Misses       11602    11598       -4     

Components	Coverage Δ
cvat-ui	77.55% <ø> (+0.05%)	⬆️
cvat-server	71.03% <100.00%> (+0.02%)	⬆️

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud