R4R: allow multiple simultaneous redelegations/ubds between same delegator/validator(s) addresses

[rigelrozanski]

closes #1402
follow up ref issue: #3270

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.
• Wrote tests
• Updated relevant documentation (docs/) - more major doc updates to come during spec upgrade
• 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 #3243 into develop will decrease coverage by <.01%.
The diff coverage is 64.86%.

@@             Coverage Diff             @@
##           develop    #3243      +/-   ##
===========================================
- Coverage    55.65%   55.64%   -0.01%     
===========================================
  Files          134      134              
  Lines         9716     9746      +30     
===========================================
+ Hits          5407     5423      +16     
- Misses        3965     3980      +15     
+ Partials       344      343       -1

[rigelrozanski]

Note this subtle change of this line to instead use of entry.IsMature(now) - previously a delegation would have been slashed if it was at MinTime (which is a bit of an ambiguous name supposed to represent "min time to completion"). Under the updates the delegation will not be slashed if now==completionTime (rename from MinTime) as this would be considered a mature bond.

[rigelrozanski]

same subtle change as previously mentioned

[cwgoes]

Mostly LGTM, tests look fine, one minor comment. We can delete ErrConflictingRedelegation in x/stake/types/errors.go.

[cwgoes]

This could use another reviewer, maybe @sunnya97 or @jaekwon.

[alexanderbez]

Left a few minor comments, but the changes look reasonable to me :)