LanderDB Performance

Hi Max.
Seems like you have performance issure in zCoin.
I think it may work pretty slow if you have a lot users in network and lots of coins mined.
I am looking into landerdb.py now, seems like when you made some changes on database you always write all coins to disk every time, all entire db.db just get overwritten every time when new coin get mined.
Now imagine if have lots of miners in the network, so "relay" nodes will get a lots of new coins per minute and so finally they will reach some hardware write limit.
I would consider some improvements on landerDB so it can just update local file instead of rewrite it completely every time. I believe current solution is not really scalable but it is easy to fix in my opinion.
I am considering using zCoin in my code so probably I will do that for you :-)