Automatic cleanup of ticker_usd

[wtogami]

Currently Baron grabs the Bitstamp vwap every 5 minutes and saves it to the ticker_usd Couch database. This data is used during subsequent transaction reception to record the approximate USD value at the time of the Bitcoin transaction, which can be helpful for accounting purposes.

The ticker_usd database currently grows slowly forever while older data is not needed. It would be helpful to have a config option that triggers automatic cleanup of data old enough that it would not be useful to keep.

Must figure out what that time duration is ... it might be extremely short for most transactions, however downtime recovery might complicate this.