Fix the coin supply calculation (#309)
Fix the coin supply calculation (#309)

* Fix the coin supply calculation

The coin supply calculation was inaccurate. It has been fixed to
exactly calculate the coin supply based on the history of the
blockchain.

* Final block exception

* <=

jcmincke/dcrd,decred/dcrd,jrick/btcd,jcmincke/dcrd,jrick/dcrd,jcmincke/dcrd,jrick/btcd,decred/dcrd,jrick/btcd,jrick/dcrd