protobuf version is not tightly pinned

[wbl]

Apparently minor version increments of protobuf package are incompatible with generated code, but it seems difficult to pin the version of the protobuf package more tightly then the grpc-rust dependency does. This then forces one to guess at the protobuf version needed to regenerate the code.

Perhaps this is due to my unfamiliarity with rust packaging, but this has wasted a good few hours of my time today. What sorts of alternatives are there?