Exonerate all transaction fees when blockchain use is low

Transaction fees are already refunded for accounts linked to an identity, but anonymous accounts still have to pay fees.

The suggestion in this forum thread is to refund fees also for anonymous accounts when blockchain use is below a certain threshold.

This could be implemented on a per-block basis (contrary to personal quotas which span a larger timescale) with a parametric value. Below threshold, no anonymous fees are paid. Above the threshold, all anonymous fees are paid.