fix how spot_ids are handled by build_traces_sequential and Label._assign()

[mattcai]

Two bug fixes:

1. Made build_traces_sequential reassign unique spot_ids to every feature after concatenating PerImageSliceResults.
2. Made Label._assign() explicitly label features by spot_id instead of row number, ensuring that labeling is accurate even when spot_ids do not match row numbers in an IntensityTable. Spot_ids must still be unique for accurate labeling though.

[codecov-io]

Codecov Report

Merging #1872 into master will decrease coverage by 31.83%.
The diff coverage is 33.33%.

@@             Coverage Diff             @@
##           master    #1872       +/-   ##
===========================================
- Coverage   87.77%   55.94%   -31.84%     
===========================================
  Files         168      168               
  Lines        5785     5786        +1     
===========================================
- Hits         5078     3237     -1841     
- Misses        707     2549     +1842     

Impacted Files	Coverage Δ
starfish/core/spots/DecodeSpots/trace_builders.py	53.33% <0.00%> (-46.67%)	⬇️
starfish/core/spots/AssignTargets/label.py	91.17% <100.00%> (ø)
starfish/morphology.py	0.00% <0.00%> (-100.00%)	⬇️
...ish/core/imagestack/parser/tilefetcher/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
...fish/core/imagestack/parser/tilefetcher/_parser.py	0.00% <0.00%> (-90.33%)	⬇️
starfish/test/full_pipelines/api/test_dartfish.py	19.67% <0.00%> (-80.33%)	⬇️
starfish/core/spots/DecodeSpots/util.py	20.40% <0.00%> (-79.60%)	⬇️
starfish/test/full_pipelines/api/test_iss_api.py	22.22% <0.00%> (-77.78%)	⬇️
...e/experiment/builder/test/factories/all_purpose.py	22.91% <0.00%> (-77.09%)	⬇️
starfish/core/starfish.py	0.00% <0.00%> (-76.93%)	⬇️
... and 87 more

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