* 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.
