Use Notify instead of stupid sleep in background worker
Use Semaphore to limit concurrent requests in rpc_client
Make more background tasks cancellable
Trying to separate:
1. Stuff for handling the swarm of nodes and generic table data replication
2. Stuff for the object store core application: metadata tables and block management
3. Stuff for the S3 API
Issue: RC increases also when the block ref entry is first put by the actual client.
At that point the client is probably already sending us the block content,
so we don't need to do a get...
We should add a delay before the task is added or find something to do.
So the HTTP client future of Hyper is not Sync, thus the stream
that read blocks wasn't either. However Hyper's default Body type
requires a stream to be Sync for wrap_stream. Solution: reimplement
a custom HTTP body type.
