* put TestCreateV0 back in StoreForTest
* avoid direct handles to V0 pieceinfo db
* type mismatch fix
* use storage.Blobs interface in store_test.go
..instead of filestore.Store. this will allow filestore.Store to become
unexported.
* unexport filestore.Store
rename it to blobStore. things should use the storage.Blobs interface
instead. changes in this commit are purely mechanical (made through the
"refactor" tool in Gocode followed by search/replace on the word "Store"
within the storage/filestore/ directory).
* kill filestore.StoreForTest
now that filestore.blobStore is unexported, there isn't a need for a
specialized wrapper type. this (not coincidentally) also makes it
possible for the WriterForFormatVersion() method on
storagenode/pieces.StoreForTest to work, without requiring everything to
wrap the store.blobs attribute in a filestore.StoreForTest, which was
impractical.
* Make the exiting node check piece hashes, piece IDs, and piece hash signatures before relaying successful transfer data to the satellite.
* Enable immediate graceful exit failure for "successful" transfers that fail satellite-side validation.
* Move transfer piece logic in storagenode worker to separate function (to make the worker easier to understand)
