only 42 coins please

This change will set a limit at 42 coins which currently doesn't exist.  It will also stretch the time before reaching 42 coins to about 7 years from now by reducing the block reward slowly, rather than next year at block 990382 which is when 42 coins will be reached if there are no reductions in block award.  This will create a hard fork at block 498330.  A hard fork at some point will be necessary if there is to be a limit at 42 coins.