Allow a fixed list of change addresses

All credit goes to @dooglus for making this change on Dogecoin. This is just something I would like to see doge in the Vertcoin client as well.


Almost a direct copy of his initial Pull https://github.com/dogecoin/dogecoin/pull/187

Almost every transaction out of a Vertcoin wallet makes a new change address in the wallet for 'change'. After a while a heavily used wallet gets very big and starts consuming all available CPU as it scans incoming transactions against an every-growing list of change addresses.

This pull request allows the user to specify a finite number of change addresses in vertcoin.conf and have transactions out of the wallet send change to one of the specified addresses at random.

In vertcoin.conf, specify 1 or more addresses, like this:

     change=Vaddress1
     change=Vaddress2
     [etc.]

If no change addresses are specified, vertcoin-qt will fall back to the previous behavior of making new change addresses per transaction.

This helps pools and coin services isolate all their change into one address which allows a a user to backup a set of private keys and not worry about loosing coins with change transactions