Index on action and start_timestamp

The action and start_timestamp index is required for queries to work correctly.

From mongo CLI
```
> db.action_execution_d_b.getIndexes()
[
	{
		"v" : 1,
		"key" : {
			"_id" : 1
		},
		"ns" : "st2.action_execution_d_b",
		"name" : "_id_"
	},
	{
		"v" : 1,
		"key" : {
			"start_timestamp" : -1
		},
		"ns" : "st2.action_execution_d_b",
		"background" : false,
		"name" : "start_timestamp_-1",
		"dropDups" : false
	},
	{
		"v" : 1,
		"key" : {
			"action" : 1
		},
		"ns" : "st2.action_execution_d_b",
		"background" : false,
		"name" : "action_1",
		"dropDups" : false
	}
]
```