Add DatagramSocket

[tzaeschke]

Implement a DatagramSocket with SCION support.

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This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 63.76022% with 133 lines in your changes are missing coverage. Please review.

Project coverage is 75.55%. Comparing base (48a03ab) to head (a6192d8).

Files	Patch %	Lines
...ain/java/org/scion/jpan/socket/DatagramSocket.java	55.21%	111 Missing and 5 partials ⚠️
...rg/scion/jpan/socket/SelectingDatagramChannel.java	82.05%	3 Missing and 4 partials ⚠️
src/main/java/org/scion/jpan/ScionService.java	71.42%	2 Missing and 2 partials ⚠️
.../java/org/scion/jpan/internal/AddressResolver.java	72.72%	1 Missing and 2 partials ⚠️
...main/java/org/scion/jpan/internal/SimpleCache.java	93.18%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #31      +/-   ##
============================================
- Coverage     77.41%   75.55%   -1.86%     
- Complexity      430      511      +81     
============================================
  Files            28       32       +4     
  Lines          2063     2426     +363     
  Branches        274      298      +24     
============================================
+ Hits           1597     1833     +236     
- Misses          349      467     +118     
- Partials        117      126       +9     

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