change peer-selection to allow introducerless explicit serverlist, alternative backends

[warner]

(note: this is no longer closely related to Accounting, since my new plan for Accounting is to send signed requests over a Foolscap channel, instead of using rights-amplification to get from an initial Foolscap object to a per-account object).

I'd like to have a section in the client's tahoe.cfg which lets it specify the servers available for storage. In contrast to #573 (which is about runtime/per-upload specification of which servers to use, out of the set provided by the introducer), this ticket is about boot-time configuration of the available set, potentially replacing the Introducer-provided list.

My thought is that the tahoe.cfg should have a section that specifies a list of servers to use. Then another tahoe.cfg setting should have a flag which says "use the Introducer to populate this list", and the default configuration would use the Introducer. This latter section would also have a place to configure the #466-style "blesser" (a pubkey which tells the client to only accept server announcements which have been signed by the matching privkey).

This would also make it possible to configure alternative server types. The first such server type I'd like to add is an S3-based server. Regular servers would be defined with a FURL; S3 servers would be defined with a service URL and a set of authorization secrets.

The syntax I'm thinking of would look like this:

[client-server-selection]
server.X.type = tahoe-foolscap
server.X.nickname = alice
server.X.furl = pb://...
server.Y.type = tahoe-foolscap
server.Y.nickname = bob
server.Y.furl = pb://
server.Z.type = s3
server.Z.nickname = aws
server.Z.url = http://...
server.Z.creds = ...
server.Z.num_shares = 3
use_introducer = False
permute_serverids = False

The server.* lines would basically define a list of dictionaries (the "X" and "Y" strings would be discarded after tahoe.cfg is parsed).

The "use_introducer=False" line means that the client shouldn't bother talking to the Introducer. If it were True, the client would connect to the introducer and add whatever servers it knew about to the list.

The "permute_serverids=False" line means that the client shouldn't permute the serverlist on each upload. Instead, it should assign 1 (or num_shares=) shares to each server in the order they appear in this list. The total-shares "N" value ought to equal the number of servers (or rather the sum of the num_shares= values).

Having permute_serverids=False in the tahoe.cfg, rather than provided on a per-upload basis (as in #573) might prove more usable. It might be more appropriate for a fairly stable grid though: one in which new servers are not being added very frequently.

[warner]

repurposes this ticket to talk about configuring the server list in tahoe.cfg, potentially instead of using the Introducer