* 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.
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