Fix EventOnly behaviour (Reprise)

[emgre]

Fix #384 (Reprise).

The fix in #385 was not enough, since it kept reporting the old static value. I added unit tests that verify both the static and the event value to make sure this fix works.

[codecov]

Codecov Report

Merging #386 into develop will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #386      +/-   ##
===========================================
- Coverage    51.72%   51.69%   -0.03%     
===========================================
  Files          456      456              
  Lines        13602    13602              
===========================================
- Hits          7036     7032       -4     
- Misses        6566     6570       +4     

Impacted Files	Coverage Δ
cpp/lib/src/outstation/StaticDataMap.h	90.65% <100.00%> (-3.74%)	⬇️

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