Don't reload unit data if nothing changed on disk when reading Campaigns

[sixlettervariables]

I noticed this while debugging this weekend, but basically we were reloading mech data even if nothing changed on disk during CPNX load. Now, this nearly always hits the hot cache and does not make a big impact for most users. However, when I moved MHQ over to a spinning disk on my machine, this shaved about 10 seconds off CPNX load times. Folks should not expect that amount of speedup in general, as most users do not have a few hundred customs (from all of the CPNXs shared on bug reports).

[codecov]

Codecov Report

Merging #2286 (e0dc4c1) into master (03ffd7d) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##             master   #2286      +/-   ##
===========================================
- Coverage      8.28%   8.28%   -0.01%     
  Complexity     2587    2587              
===========================================
  Files           673     673              
  Lines         95085   95087       +2     
  Branches      15983   15984       +1     
===========================================
  Hits           7876    7876              
- Misses        86142   86144       +2     
  Partials       1067    1067              

Impacted Files	Coverage Δ	Complexity Δ
MekHQ/src/mekhq/campaign/io/CampaignXmlParser.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)

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