optional to enable tracing for linkerd/namerd/namer/tracing requests

[siggy]

linkerd currently disables tracing for its own internal requests involving linkerd, namerd, namers, and tracing. the intent is to keep a user's zipkin or tracing data strictly for their own service requests.

however, there are cases where tracing for these internal requests is useful. specifically, for debugging linkerd/namerd issues. propose adding a config option to enable tracing for internal requests. this change would probably start with a search for all calls to Trace.letClear.