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

scripts/simulation-after-import.sh

@@ -0,0 +1,54 @@
+#!/bin/bash
+
+seeds=(1 2 4 7 9 20 32 123 124 582 1893 2989 3012 4728 37827 981928 87821 891823782 989182 89182391 \
+11 22 44 77 99 2020 3232 123123 124124 582582 18931893 29892989 30123012 47284728 37827)
+blocks=$1
+
+echo "Running multi-seed import-export simulation with seeds ${seeds[@]}"
+echo "Running $blocks blocks per seed"
+echo "Edit scripts/simulation-after-import.sh to add new seeds. Keeping parameters in the file makes failures easy to reproduce."
+echo "This script will kill all sub-simulations on SIGINT/SIGTERM (i.e. Ctrl-C)."
+
+trap 'kill $(jobs -pr)' SIGINT SIGTERM
+
+tmpdir=$(mktemp -d)
+echo "Using temporary log directory: $tmpdir"
+
+sim() {
+  seed=$1
+	echo "Running simulation after import with seed $seed. This may take awhile!"
+  file="$tmpdir/gaia-simulation-seed-$seed-date-$(date -Iseconds -u).stdout"
+  echo "Writing stdout to $file..."
+  go test ./cmd/gaia/app -run TestGaiaSimulationAfterImport -SimulationEnabled=true -SimulationNumBlocks=$blocks \
+    -SimulationBlockSize=200 -SimulationCommit=true -SimulationSeed=$seed -v -timeout 24h > $file
+}
+
+i=0
+pids=()
+for seed in ${seeds[@]}; do
+  sim $seed &
+  pids[${i}]=$!
+  i=$(($i+1))
+  sleep 10 # start in order, nicer logs
+done
+
+echo "Simulation processes spawned, waiting for completion..."
+
+code=0
+
+i=0
+for pid in ${pids[*]}; do
+  wait $pid
+  last=$?
+  seed=${seeds[${i}]}
+  if [ $last -ne 0 ]
+  then
+    echo "Import/export simulation with seed $seed failed!"
+    code=1
+  else
+    echo "Import/export simulation with seed $seed OK"
+  fi
+  i=$(($i+1))
+done
+
+exit $code