grpc: Convert key to a bytes field

Galaxycache is perfectly fine with non-UTF-8 keys, but some go Protobuf
library versions validate UTF-8 conformance when serializing
string-fields.

We can side-step this by setting the field definitions to conform to the
protobuf spec by putting our possibly-not-utf-8 strings in a bytes
field.

Fortunately, string and bytes fields are 100% wire compatible, so
switching the type only creates a mild API break.

I doubt anyone's using these protobuf messages directly, so I'm just
going to declare that they aren't covered by v1.x compatibility.

Also, regenerate the protobuf types with the current protobuf library
(google.golang.org/protobuf instead of github.com/golang/protobuf).

Include a test to guarantee that this does the right thing with invalid
UTF-8.