Add a flag to export for zero-height start (#2827)

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

docs/examples/basecoin/cmd/basecoind/main.go

@@ -124,7 +124,7 @@ func newApp(logger log.Logger, db dbm.DB, storeTracer io.Writer) abci.Applicatio
 	return app.NewBasecoinApp(logger, db, baseapp.SetPruning(viper.GetString("pruning")))
 }
 
-func exportAppStateAndTMValidators(logger log.Logger, db dbm.DB, storeTracer io.Writer, _ int64) (
+func exportAppStateAndTMValidators(logger log.Logger, db dbm.DB, storeTracer io.Writer, _ int64, _ bool) (
 	json.RawMessage, []tmtypes.GenesisValidator, error) {
 	bapp := app.NewBasecoinApp(logger, db)
 	return bapp.ExportAppStateAndValidators()

docs/examples/democoin/cmd/democoind/main.go

@@ -128,7 +128,7 @@ func newApp(logger log.Logger, db dbm.DB, _ io.Writer) abci.Application {
 	return app.NewDemocoinApp(logger, db)
 }
 
-func exportAppStateAndTMValidators(logger log.Logger, db dbm.DB, _ io.Writer, _ int64) (
+func exportAppStateAndTMValidators(logger log.Logger, db dbm.DB, _ io.Writer, _ int64, _ bool) (
 	json.RawMessage, []tmtypes.GenesisValidator, error) {
 	dapp := app.NewDemocoinApp(logger, db)
 	return dapp.ExportAppStateAndValidators()