Closes#2812
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
* Update slashing import/export
* More slashing.WriteGenesis
* Add test import/export to CI
* Store equality comparison.
* Fix validator bond intra-tx counter
* Set timeslices for unbonding validators
* WriteGenesis => ExportGenesis
* Delete validators from unbonding queue when re-bonded
* Hook for validator deletion, fix staking genesis tests
Christopher Goes
Alexander Bezobchuk
Jae Kwon