Transaction proving takes too long to run in RPCServer

[Sword-Smith]

When initiating a transaction through the dashboard on a machine that produces SingleProof pressing Send crashes the dashboard because RPCServer is blocked for the few minutes it takes to produce the proof.

The best solution is probably to produce a PrimitiveWitness data structure in the RPCServer and then send this to the main_loop that then needs to spawn a new task to produce the proof.