* 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.
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.
