fairness: update headers of source files

[cmoussa1]

The headers for some of the files in the fairness directory need to be updated to the new minimal header and made consistent with the other files in this repo.

This PR updates the headers of some of the files in the fairness directory.

[codecov]

Codecov Report

Merging #113 (f7478a7) into master (c8cec66) will not change coverage.
The diff coverage is n/a.

❗ Current head f7478a7 differs from pull request most recent head 693733a. Consider uploading reports for the commit 693733a to get more accurate results

@@           Coverage Diff           @@
##           master     #113   +/-   ##
=======================================
  Coverage   85.37%   85.37%           
=======================================
  Files          13       13           
  Lines         718      718           
=======================================
  Hits          613      613           
  Misses        105      105           

Impacted Files	Coverage Δ
src/fairness/account/account.cpp	60.60% <ø> (ø)
src/fairness/account/account.hpp	100.00% <ø> (ø)
src/fairness/reader/test/data_reader_db_test01.cpp	98.52% <ø> (+0.02%)	⬆️
...fairness/weighted_tree/test/weighted_tree_load.cpp	77.55% <ø> (ø)
...irness/weighted_tree/test/weighted_tree_test01.cpp	100.00% <ø> (ø)
src/fairness/weighted_tree/weighted_tree.cpp	86.79% <ø> (ø)
src/fairness/weighted_tree/weighted_tree.hpp	100.00% <ø> (ø)
src/fairness/weighted_tree/weighted_walk.cpp	95.41% <ø> (ø)
src/fairness/weighted_tree/weighted_walk.hpp	100.00% <ø> (ø)
... and 1 more

[dongahn]

LGTM.

[cmoussa1]

Thanks @dongahn!

[cmoussa1]

I'll go ahead and set MWP.