ENH: Optimize graph fusion

[aresnow1]

What do these changes do?

Optimize the coloring algorithm to increase task parallelism.

Related issue number

Fixes #384

Check code requirements

• [ ] tests added / passed (if needed)
• [ ] Ensure all linting tests pass

[codecov[bot]]

Codecov Report

Merging #609 (8a7001e) into main (4d985a2) will decrease coverage by 0.11%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #609      +/-   ##
==========================================
- Coverage   93.59%   93.48%   -0.11%     
==========================================
  Files        1012     1012              
  Lines       78729    78738       +9     
  Branches    16358    16361       +3     
==========================================
- Hits        73687    73611      -76     
- Misses       3387     3459      +72     
- Partials     1655     1668      +13     

Flag	Coverage Δ
unittests	93.38% <100.00%> (-0.11%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...n/xorbits/_mars/services/task/analyzer/analyzer.py	93.90% <ø> (ø)
...hon/xorbits/_mars/services/task/analyzer/fusion.py	100.00% <100.00%> (ø)

... and 18 files with indirect coverage changes