Fix opentracing setup for gitlab-sshd
Previously, opentracing (if configured) was initialized late in the gitlab-shell process's lifespan, coming just before making a gRPC call to Gitaly.
By moving the opentracing initialization to be at process startup, we make it available for the whole process lifecycle, which is very useful to gitlab-sshd, as it means we'll only call tracing.Initialize() once on process startup, rather than once per SSH connection.
To get this working, we need to introduce a context to gitlab-sshd. This carries the client/service name, but also carries an initial correlation ID. The main outcome of this is that all calls to the authorized_keys endpoint from a given gitlab-sshd process will now share a correlation ID. I don't have a strong opinion about this either way.
Closes #501 (closed)