[R4R] simulation: Make validator choice use validator set

[ValarDragon]

This also had to change the default seed, since with the previous one it
actually got into a state where there were no validators left bonded, lol.

This also changes Unbond msgs from failing with almost 100% probability to now
only failing with 33% probability.
Thus more of the state machine is getting tested!

This does slow it down a fair amount (went from 500 blocks in ~55 seconds, to now 500 blocks in 100 seconds) but I think this is a fair trade-off since now its actually testing these, whereas before it wasn't.

Before:

stake/MsgDelegate/false => 650
stake/MsgDelegate/true => 2896
stake/MsgBeginUnbonding/false => 3481
stake/MsgBeginUnbonding/true => 70

After:

stake/MsgDelegate/false => 291
stake/MsgDelegate/true => 2864
stake/MsgBeginUnbonding/false => 1156
stake/MsgBeginUnbonding/true => 2181

Closes #2621

• Targeted PR against correct branch (see CONTRIBUTING.md)

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.

• Wrote tests - sim is the test

• 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 #2660 into develop will decrease coverage by 0.04%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           develop    #2660      +/-   ##
===========================================
- Coverage    58.82%   58.77%   -0.05%     
===========================================
  Files          152      152              
  Lines         9425     9433       +8     
===========================================
  Hits          5544     5544              
- Misses        3511     3519       +8     
  Partials       370      370

[alexanderbez]

All for this! Good summary. Thanks @ValarDragon .

testedACK

[cwgoes]

utACK