Add mediator tests and clean up

[didiergarcia]

Add a bunch of test for Mediator and clean up a few things in it's implementation.

[codecov-commenter]

Codecov Report

Patch coverage: 99.14% and project coverage change: +0.88 🎉

Comparison is base (dcc140b) 78.30% compared to head (3dfa71c) 79.19%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #143      +/-   ##
============================================
+ Coverage     78.30%   79.19%   +0.88%     
- Complexity      491      504      +13     
============================================
  Files            73       74       +1     
  Lines          6029     6258     +229     
  Branches        757      760       +3     
============================================
+ Hits           4721     4956     +235     
+ Misses          711      703       -8     
- Partials        597      599       +2     

Impacted Files	Coverage Δ
...ent/analytics/kotlin/core/platform/MediatorTest.kt	99.12% <99.12%> (ø)
...segment/analytics/kotlin/core/platform/Mediator.kt	96.66% <100.00%> (+10.45%)	⬆️
...segment/analytics/kotlin/core/platform/Timeline.kt	90.90% <100.00%> (ø)
...n/java/com/segment/analytics/kotlin/core/Events.kt	47.42% <0.00%> (+2.57%)	⬆️

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