This patch was originally written by Brian, but was re-recorded by Zooko to use
darcs replace instead of hunks for any file in which it would result in fewer
total hunks.
refs #1363
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.
