Deprecate the pieceinfo database, and start storing piece info as a header to
piece files. Institute a "storage format version" concept allowing us to handle
pieces stored under multiple different types of storage. Add a piece_expirations
table which will still be used to track expiration times, so we can query it, but
which should be much smaller than the pieceinfo database would be for the
same number of pieces. (Only pieces with expiration times need to be stored in piece_expirations, and we don't need to store large byte blobs like the serialized
order limit, etc.) Use specialized names for accessing any functionality related
only to dealing with V0 pieces (e.g., `store.V0PieceInfo()`). Move SpaceUsed-
type functionality under the purview of the piece store. Add some generic
interfaces for traversing all blobs or all pieces. Add lots of tests.
* 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
