* add cache, update cache w/piece create/delete
* add service w/loop to cache to recalculate space used cache
* add piecestore cache to other sn svcs to use
* add table to persist the total space used
* rm cache where not needed
* rm stuff from sn svcs
* start fixing tests, changes per comments
* update commits
* add unit tests
* fix commiting before we write header bytes
* fix cache create test
* copy cache map, add started back to recalc
* fix test
* add test, update comments
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.
