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
* Add arg to PrintUnsignedStdTx() to actually operate in offline mode
* WIP simplifying server module
* Expose ExternalIP()
* Move {GenTx,Init}Cmd into gaia's new init package
Merges the keybase and Ledger code from go-crypto (which is no more) into the SDK
Adds support for Ledger into gaiacli
Cherry-picks updated error handling from #1158
Alexander Bezobchuk
Alessio Treglia
Jack Zampolin
Christopher Goes
Ethan Buchman
Christopher Goes
rigelrozanski