cache per instance
Credentials, Stats, getAllSecrets

 - Credentials class to encapsulate the datails to access a container
 - Stats class to account server stats and log store/server stats
 - getAllSecrets for getting a dict with all containers credentials
Use new Credentials class
Serve all the containers specified in secrets file

- Use new Credentials class
- Specifying a container is now not required
- Server requires a dict of storages indexed by container name
- Use Server.unlock_all() on exit
Support for several containers

- Use the new Stats class
- Track stats per container, log stats per container
- Storage negotiation (NBD_OPT_EXPORTNAME)
- Support for listing exports (NBD_OPT_LIST)