Adds AtomicAge to bucket map holder

[brooksprumo]

Problem

I'm investigating making the accounts index flushing interval longer, which will need to change the Age type from a u8 to something bigger. There is an atomic and non-atomic set of types for the Age. They are used in multiple places, and it would be safer to ensure they are always the same.

Summary of Changes

Add an AtomicAge alias, and assert it's the same size as Age.

[codecov]

Codecov Report

Merging #33841 (4f4e69a) into master (b0dcaf2) will increase coverage by 0.0%.
Report is 10 commits behind head on master.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##           master   #33841   +/-   ##
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  Coverage    81.8%    81.9%           
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  Files         809      809           
  Lines      217643   217627   -16     
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  Hits       178247   178247           
+ Misses      39396    39380   -16     

[HaoranYi]

lgtm