Fix non-daemon-threads in the JDK poller and avoid polluting the commonPool

[DavyLandman]

This PR fixes #57 and also changes the default thread pool that is used when users do not provide their own.

This way commonPool is not polluted by long running jobs.

[codecov]

Codecov Report

Attention: Patch coverage is 88.23529% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.0%. Comparing base (24af885) to head (f433eb2).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/main/java/engineering/swat/watch/Watch.java	75.0%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #59     +/-   ##
=========================================
+ Coverage     81.6%   82.0%   +0.4%     
- Complexity     164     169      +5     
=========================================
  Files           23      24      +1     
  Lines          828     841     +13     
  Branches        97      98      +1     
=========================================
+ Hits           676     690     +14     
  Misses          92      92             
+ Partials        60      59      -1     

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