Code review comment for lp:~robertcarr/mir/implement-client-credentials

Whoops the acceptance test was missed (bzr add).

Of course the pid is retreived from the socket. The interface of mf::SessionMediator however is generated by protoc and it's difficult to modify without encoding messages in the IPC level. So, we use the same pattern we use for FDs, encode them in the protobuf as optional fields, and have the message processor fill them in using out of band data.