Load ethereum private key

Load an ethereum keystore when the node is started in coordinator mode.  The
private key corresponding to the forger address must be imported into the
keystore before running the node in coordinator mode.  You can see an examples
in `cli/node/load-sk-example.sh`.

db/statedb/statedb_test.go

@@ -112,6 +112,10 @@ func TestNewStateDBIntermediateState(t *testing.T) {
 	sdb, err = NewStateDB(dir, 128, TypeTxSelector, 0)
 	assert.NoError(t, err)
 
+	bn, err = sdb.db.GetCurrentBatch()
+	assert.NoError(t, err)
+	assert.Equal(t, common.BatchNum(1), bn)
+
 	v, err = sdb.db.DB().Get(k0)
 	assert.NoError(t, err)
 	assert.Equal(t, v0, v)