[tahoe-dev] signed Introducer messages, server-selection work

[Brian Warner]

I've been thinking about server-selection again this week, partly
because I'm no longer strictly blocked on pycryptopp for my ECDSA
primitive[1], partly because I'm a bit stuck on my new-downloader
work[2]. The relevant tickets are #466, #295, #467, and #68.

The basic topic is this: how does a given client decide which servers to
use? The current behavior is "use whatever the Introducer told you
about", but we want to make that more flexible. The Introducer's job is
mostly just distribution of information, but the current implementation
also gives it the authority to decide what servers will be used by all
clients.

To enable a distributed Introducer (i.e. via gossip-based flooding of
announcements), we need to separate these two concerns. The Introducer
should be just plumbing, responsible for efficiently distributing
information about potential servers. The "which of those servers should
we use" authority should be expressed in some other fashion.
Fundamentally, this authority comes from the client, but it should be
easy for the client admin to delegate this decision to someone else.