Change maximum interest rate increase/decrease

Change the maximum interest rate rise from 1% per 1440 blocks to a per block rise of 0.002%. Enforce a maximum per block interest rate decrease of 0.004%.

Issue #166 was marked as a duplicate of this issue.

The motion: https://discuss.nubits.com/t/passed-finalized-motion-for-protocol-changes/372

Solution proposed in pull request #221.

Pull request merged.