This will make it possible to use Processor::getProof(proofid) in
D9370 to get any proof, not only the proof used by the local node.
It now returns a pointer. This avoids doing a copy of the proof and
having to catch an error to find out that there is none (instead we can
just check for nullptr). It will solve the copy issue mentioned in the review of D9368.
A pointer is necessary because C++ does not seem to allow optional references.