R4R: Add a flag to export for zero-height start

[cwgoes]

Closes #2812

Depends on #2807 and #2825, can be retargeted to develop once those PRs are merged.

This PR adds the flag --for-zero-height to gaiad export, which runs several alterations to the application state to prepare for restarting a new chain in a consistent fashion.

It also:

• Moves Gaia's export code to cmd/gaia/app/export.go for cleaner separation.
• Fixes an inconsistency where we treated the initChainer as happening at height -1 - it should now happen at height 0, since the first header sent by Tendermint has height 1.
• Runs the runtime invariant checks on start (in initChainer)
• Adds a few auxiliary functions to clear slashing periods
• Removes the Height field from Delegation objects in x/stake, which was not used anywhere

Standard checklist:

• 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

❗ No coverage uploaded for pull request base (develop@7cb1ba6). Click here to learn what that means.
The diff coverage is 34.78%.

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[cwgoes]

Tendermint used to start at block height 0, now it starts at 1, so we needed to update this.

[rigelrozanski]

woot! - few comments herein - Also did a push for some formatting and a missing Close()

[cwgoes]

Retargeted on #2825 for the new invariants.

[rigelrozanski]

Thanks Chris!

[cwgoes]

OK, there's a latent distribution oddity somewhere. It isn't introduced by this PR, but it seems to have been discovered only now. I've added the invariant to the general simulation, which now fails on some seeds.

[rigelrozanski]

@cwgoes you should test a merge on top of some of the bugfixes in #2825 - could easily be related as those bugfixes related to distribution

[jaekwon]

To debug this, I'm improving the simulator a bit...
#2897

@rigelrozanski I've verified on my branch above that the bugfixes do not address chris's new bug. NOTE: chris's new invariance has been added to the above PR. It changes usage of rand, so we'll need to find new seeds that cause the problem.