data/DiffStore: auto-create the rstore if its just a config

data/DiffStore: auto-create the rstore if its just a config

when rstore is not instanciated but only a config,
auto-create it with its type

this allows us to configure an diff/rstore combination completely
declaratively like this:

store: {
    type: 'diff',
    autoDestroy: true,
    autoDestroyRstore: true,
    rstore: {
	type: 'update',
	model: 'some-model',
	autoStart: true,
	interval: 5000,
    },
},

the only thing we have to be careful about is to either
do a manual 'stopUpdate' somewhere, or use the 'autoDestroyRstore' flag

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>