R4R: Block conflicting redelegations

[cwgoes]

Quick fix to #1402 for GoS, but we should eventually add an index instead.

Simply prevents redelegation by delegator D from validator A to validator B if any existing redelegation by D from A to B already exists. Includes a testcase.

• 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

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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 #2500 into develop will decrease coverage by <.01%.
The diff coverage is 33.33%.

@@             Coverage Diff             @@
##           develop    #2500      +/-   ##
===========================================
- Coverage    61.38%   61.37%   -0.01%     
===========================================
  Files          153      153              
  Lines         9612     9615       +3     
===========================================
+ Hits          5900     5901       +1     
- Misses        3301     3302       +1     
- Partials       411      412       +1

[rigelrozanski]

Got it - thanks for the patch