MultiMap clean up

[tzaeschke]

Test and clean up MultiMap

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This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 74.81%. Comparing base (dff4a70) to head (a843033).

Files	Patch %	Lines
src/main/java/org/scion/internal/Segments.java	50.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #34      +/-   ##
============================================
+ Coverage     74.51%   74.81%   +0.29%     
- Complexity      353      356       +3     
============================================
  Files            26       26              
  Lines          1766     1759       -7     
  Branches        228      227       -1     
============================================
  Hits           1316     1316              
+ Misses          342      336       -6     
+ Partials        108      107       -1     

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