Calculate yield based on avg expected vote height.
Calculate yield based on avg expected vote height.

This modifies the expected yield calculation to use the per vote subsidy
for the height that is 28 days in the future since that is the expected
average time before a ticket will vote.

This is slightly more accurate in general and it allows the expected
yield to be properly calculated before voting begins where the per vote
subsidy is 0 which leads to an inaccurate expected yield of 0%.

davecgh/dcrstakesim,raedah/dcrstakesim