refactor: replace BFS with DFS in sync_txn_request

[bsbds]

Close: #498
Please briefly answer these questions:

• what problem are you trying to solve? (or if there's no problem, what's the motivation for this change?)

• what changes does this pull request make?

• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc)

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (fe5b7de) 54.17% compared to head (1576dd9) 54.19%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #504      +/-   ##
==========================================
+ Coverage   54.17%   54.19%   +0.01%     
==========================================
  Files         101      101              
  Lines       18239    18237       -2     
  Branches    18239    18237       -2     
==========================================
+ Hits         9881     9883       +2     
+ Misses       7768     7765       -3     
+ Partials      590      589       -1     

Files	Coverage Δ
xline/src/server/command.rs	68.98% <0.00%> (-0.11%)	⬇️
xline/src/storage/kv_store.rs	71.90% <66.66%> (-0.19%)	⬇️

... and 1 file with indirect coverage changes

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[bsbds]

After refactoring this PR is no longer needed.