R4R: Implement GenesisState.Sanitize()

[alessio]

Implement genesis file sanitization to avoid failures at chain init.

• Add GenesisState.Sanitize(). It sorts genesis accounts and
  coin sets to ensure genesis state passes validation.
• gaia app calls GenesisState.Sanitize() on initFromGenesisState()
  before processing the genesis state.

Closes: #3390

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.
• Wrote tests
• Updated relevant documentation (docs/)
• Added entries in PENDING.md with issue #
• rereviewed Files changed in the github PR explorer

---

For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #3397 into develop will decrease coverage by 0.01%.
The diff coverage is 50%.

@@             Coverage Diff             @@
##           develop    #3397      +/-   ##
===========================================
- Coverage    59.38%   59.37%   -0.02%     
===========================================
  Files          131      131              
  Lines         9862     9868       +6     
===========================================
+ Hits          5857     5859       +2     
- Misses        3632     3637       +5     
+ Partials       373      372       -1

[jackzampolin]

Great additions here. I held comments cause they were all ++

[alexanderbez]

mostly LGTM, left a few minor remarks though 👍

[rigelrozanski]

LGTM