Rewrite of left-right protocol

[Rob Austein <sra@hactrn.net>]

http://subvert-rpki.hactrn.net/docs/left-right-protocol-message-passing

Per previous discussion: the left-right protocol had waaay too many
operations and was much too chatty.  I've rewritten it in a form that
vaguely resembles a message-passing system: the basic messages are
about objects, each kind of object takes the same five basic actions,
and in most cases it should take only one or two round trips to set up
any given object.

I think this should translate relatively cleanly into XML.  I've left
it as s-expressions for the moment to avoid obscuring the protocol
changes under XML syntactic noise.

Comments welcome.