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.
* Added a gc package at satellite/gc, which contains the gc.Service, which runs garbage collection integrated with the metainfoloop, and the gc PieceTracker, which implements the metainfo loop Observer interface and stores all of the filters (about which pieces are good) for each node.
* Added a gc config located at satellite/gc/service.go (loop disabled by default in release)
* Creates bloom filters with pieces to be retained inside the metainfo loop
* Sends RetainRequests (or filters with good piece ids) to all storage nodes.
