Support one party owning several key shares

[fjarri]

This, of course, can be emulated by one party maintaining several separate Session objects, but it is inefficient, since then it will have to effectively creates proofs for itself. Also it would be nice to bundle messages from that party into one message to reduce network latency.

We will need some kind of a ShareDistribution object (should it be separate, or a part of KeyShare?) that Session will be able to make use of.