interest rate refactor
interest rate refactor

cancel out fterms and ninterestblockterm to nblockdifference


Former-commit-id: a267efa80a5961d4b1090eb6c480f69cfee63f6e
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin