SturdyRef replication

This is User:Kevin Reid's idea for implementing transparent load-balancing replication for services exposed over Pluribus.

Premises
We have a set of hosts running replicas of the actual service (ignoring their internal communication topology), and (possibly identical) a set of hosts which are exposed to the Internet and provide some mechanism (e.g. round robin DNS) for letting a client connect to an arbitrary one of them.

Implementation
Each of the exposed hosts is running a 'pseudo-vat'; all pseudo-vats have the same VatID. The clients hold ordinary SturdyRefs referring to this VatID.

A pseudo-vat implements CapTP trivially: it ignores all incoming messages except nonceLocator. A successful lookup returns a live reference to a presence of the relevant object in one of the replica-vats.

In order to preserve transparency, the pseudo-vat must not support any feature (e.g. incoming 3Descs, WormholeOp, etc.) which would require it to connect to other vats, as then its non-singular nature could be revealed.