Profile disk use

Current tests on ĞDev show a significant disk use: more than 50 GB per year (https://forum.duniter.org/t/soucis-potentiel-despace-disque-necessaire-pour-la-v2/12780).

We should know more about what causes disk use and do some simulations on the effect of web of trust and transfers to see if the no-fee model can be sustainable.