change peer-selection to prepare for rights-amplification step, alternative backends

[warner]

Another prep-task for the upcoming Accounting work is to change the client's peer-selection code. The goals are:

• the set of storage servers that are considered for peer-selection should come from a config file, with one server per line, of various types
• one entry type means "ask the introducer", perhaps with some parameters like "only accept servers that are blessed by the following key"
• if this file doesn't exist, it defaults to asking the introducer
• one entry type (or introducer tag) is used to point at a storage server's "login FURL" instead of a fully-powered reference. (we need a better name for this object). This is the object to which the client will pass credentials and/or signed certificate chains.

When the peer-selection process needs server references, the client should ask the login FURLs for server references, performing credential negotiation if necessary. The client should cache the results for later use, so ideally the negotiation only needs to take place once.