Refactor StorageFarmBroker handling of servers

Refactor StorageFarmBroker handling of servers

Pass around IServer instance instead of (peerid, rref) tuple. Replace
"descriptor" with "server". Other replacements:

 get_all_servers -> get_connected_servers/get_known_servers
 get_servers_for_index -> get_servers_for_psi (now returns IServers)

This change still needs to be pushed further down: lots of code is now
getting the IServer and then distributing (peerid, rref) internally.
Instead, it ought to distribute the IServer internally and delay
extracting a serverid or rref until the last moment.

no_network.py was updated to retain parallelism.