* Add in-memory cache for bandwidth and space usage monitoring
* moved some structs around and added error handling for get piece size query
* added to existing bandwidth test. fixed typo
* added test, updates from PR review, added monkit for new methods
* PR review updates. renamed space used methods
* changed bw cache so that only Add updates the cache and it only overwrites when the date moves forward
* moved bandwidth usage to bw and space usage to pieceinfodb
* fixed interface comment
* removed pointer from sync.Once
