feature: weighted serve calls

[rustatian]

Reason for This PR

closes: roadrunner-server/roadrunner#1452

Description of Changes

• Sort vertices by their weight for the serve function calls.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the MIT license.

PR Checklist

[Author TODO: Meet these criteria.]
[Reviewer TODO: Verify that these criteria are met. Request changes if not]

• All commits in this PR are signed (git commit -s).
• The reason for this PR is clearly provided (issue no. or explanation).
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this PR.
• Any user-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.

[codecov]

Codecov Report

Merging #135 (2f7e53a) into master (780bdb9) will increase coverage by 0.39%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #135      +/-   ##
==========================================
+ Coverage   74.80%   75.19%   +0.39%     
==========================================
  Files          19       19              
  Lines         893      899       +6     
==========================================
+ Hits          668      676       +8     
+ Misses        195      193       -2     
  Partials       30       30              

Impacted Files	Coverage Δ
serve.go	100.00% <100.00%> (ø)
graph/vertex.go	100.00% <0.00%> (+25.00%)	⬆️

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