Switch to python multithreading

[ttung]

smFISH notebook is ~2% faster. Didn't benchmark anything else though.

Test plan: passes unit tests, used the -alltest magic branch name introduced in #1543
Depends on: #1541, #1542, #1543
Fixes #834, #1268

[codecov-io]

Codecov Report

Merging #1544 into tonytung-preserve-float-range will decrease coverage by 0.48%.
The diff coverage is 100%.

@@                        Coverage Diff                        @@
##           tonytung-preserve-float-range    #1544      +/-   ##
=================================================================
- Coverage                          87.67%   87.18%   -0.49%     
=================================================================
  Files                                140      138       -2     
  Lines                               5045     5002      -43     
=================================================================
- Hits                                4423     4361      -62     
- Misses                               622      641      +19

Impacted Files	Coverage Δ
...re/spots/DetectPixels/combine_adjacent_features.py	98.18% <100%> (+13.63%)	⬆️
starfish/core/imagestack/imagestack.py	94.42% <100%> (-0.07%)	⬇️
starfish/core/imagestack/_mp_dataarray.py	0% <0%> (-100%)	⬇️
starfish/core/image/Filter/clip.py	100% <0%> (+3.84%)	⬆️
starfish/core/image/Filter/match_histograms.py	97.05% <0%> (+8.82%)	⬆️
starfish/core/image/Filter/linear_unmixing.py	100% <0%> (+26.92%)	⬆️

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[shanaxel42]

everything is so much simpler!