Add winery backend

Assuming there exist an object storage as described in T3054, there needs to be an object storage backend to store / retrieve objects. The name is winery (using the "winery namespace" agreed on on the mailing list last week). The implementation is a replacement of the object storage at the bottom of the global architecture.

The backend runs in a *sgi app. At boot time it:

• Gets an exclusive lock on a Shard in the Write Storage (creating a new one or resuming an existing one)
• Gets a connection to the Write Storage and the global index
• Registers for throttling purposes (probably a table in the database)
• Serves both reads and writes from and to the Read Storage and the Write Storage

When the Shard it is responsible for in the Write Storage is full:

• It stops accepting requests
• Writes the Shard to the Read Storage
• Shutdown

Spawning the *sgi app or controlling the number of workers is not in the scope of this implementation.

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Migrated from T3432 (view on Phabricator)