Reload opentracing config on SIGHUP

[DMRobertson]

We have just had an example on matrix.org where we wanted to add a new user to force_tracing_for_users to profile performance problems. We can't do this without restarting workers. It'd be great if we can just ask Synapse to reload things nicely with a SIGHUP.

[DMRobertson]

Like #12373. I am hoping we can reuse bits of the implementation (#12673) here.