* pkg/pg: Add new service function storage node
Add a new service function to the storage node piece store for deleting
pieces when satellites request them.
* storagenode/piecestore: Add endpoint to delete piece
Add a new endpoint to receive from trusted satellites to delete a piece.
* private/testplanet: Fix storagenode mock
Add to the storagenode mock the new endpoint method.
* proto.lock: Update it with the last protbuff changes
* storagenode/piecestore: Reuse test piece upload
Extract the repeated logic from several tests functions for uploading a
test piece to a test helper function.
* uplink/piecestore: Implement client side method
Implement the client side method of the new piecestore RPC function.
* storagenode/piecestore: Add test DeletePiece endpoint
Implement a test for the DeletePiece new endpoint method.
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.
