a8n: Only create new Changesets if they can be synced

Created by: mrnugget

This fixes #6535 by extending the transaction to also include the syncing of the to-be-created changesets.

With that, the transaction rolls back and doesn't create potentially invalid changesets in case one of them cannot be synced.

That is much better than adding invalid state to the database which then leads to the changesets syncer failing to sync changesets every time.

Test plan:

Created by: codecov[bot]

Codecov Report

Merging #6596 into master will decrease coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6596      +/-   ##
==========================================
- Coverage   39.19%   39.19%   -0.01%     
==========================================
  Files        1215     1215              
  Lines       62346    62345       -1     
  Branches     5935     5935              
==========================================
- Hits        24436    24435       -1     
  Misses      35672    35672              
  Partials     2238     2238

Impacted Files	Coverage Δ
enterprise/pkg/a8n/resolvers/resolver.go	38.61% <100%> (-0.25%)