Fix JOSM bug 11841

[don-vip]

Use ThreadPoolExecutor.DiscardPolicy + code cleanup/refactoring
See https://josm.openstreetmap.de/ticket/11841

[codecov-io]

Codecov Report

Merging #39 into master will increase coverage by 0.03%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master      #39      +/-   ##
============================================
+ Coverage     27.39%   27.42%   +0.03%     
  Complexity      339      339              
============================================
  Files            75       75              
  Lines          3691     3686       -5     
  Branches        484      480       -4     
============================================
  Hits           1011     1011              
+ Misses         2572     2567       -5     
  Partials        108      108

Impacted Files	Coverage Δ	Complexity Δ
.../plugins/mapillary/gui/MapillaryHistoryDialog.java	0% <ø> (ø)	0 <0> (ø)	❌
.../plugins/mapillary/MapillaryLocationChangeset.java	61.11% <ø> (ø)	4 <0> (ø)	❌
...m/plugins/mapillary/gui/MapillaryFilterDialog.java	0% <ø> (ø)	0 <0> (ø)	❌
...ins/mapillary/io/download/MapillaryDownloader.java	18.47% <ø> (ø)	0 <0> (ø)	❌
...osm/plugins/mapillary/gui/MapillaryMainDialog.java	0% <ø> (ø)	0 <0> (ø)	❌
...download/MapillarySquareDownloadManagerThread.java	0% <0%> (ø)	0 <0> (ø)	❌
...reetmap/josm/plugins/mapillary/MapillaryLayer.java	21.52% <0%> (ø)	20 <0> (ø)	❌
...treetmap/josm/plugins/mapillary/mode/JoinMode.java	0% <0%> (ø)	0 <0> (ø)	❌

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