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
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