can now update db... with no index

You can now update the db and get balances for every address. However, it is
quite slow. The slowness could be due to the huge number of queries to bitcoind,
since there is one query for every output of a transaction. Or the slowness
could be due to the fact that there is no index defined on the mongodb. Since
there are a lot of reads to the db, I suspect that is the most likely
possibility. This could be sped up substantially by putting an index on the db,
making reads to the diff collection faster. Right now I think it has to
constantly scan and sort the db on every read, which is slow. An index would
probably speed it up by a large factor, about equal to the size of the db.