Fixed flushing of skipped files

[tieneupin]

• Removed 'label' field from the 'RsyncerSource' Pydantic models as it is no longer needed
• Removed the key from requests to endpoints that use that model
• Omit the 'no rsync process ran for files' error message if files weren't scheduled for transfer to begin with
• Update the database with a new timestamp if it's later than the currently stored one when a flush is triggered
• The Rsyncer end time updates are now set by the instrument server, and not as part of the flush
• Store the time difference between the client and the server as an attribute in the MultigridController and convert incoming visit end time updates to client-side time

[codecov]

Codecov Report

Attention: Patch coverage is 4.00000% with 24 lines in your changes missing coverage. Please review.

Project coverage is 31.81%. Comparing base (4d783a7) to head (e1a9cf7).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #621      +/-   ##
==========================================
- Coverage   31.86%   31.81%   -0.06%     
==========================================
  Files          90       90              
  Lines       10762    10776      +14     
  Branches     1422     1426       +4     
==========================================
- Hits         3429     3428       -1     
- Misses       7215     7230      +15     
  Partials      118      118              

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