Clear the list of traces

[lenko-d]

This PR adds the ability to clear the list of traces.

Updates: 2269

[klizhentas]

@lenko-d how is this different from trace.Unwrap that returns you the original error?

[lenko-d]

@lenko-d how is this different from trace.Unwrap that returns you the original error?

The problem is that Unwrap changes the structure of the API's response considerably. Before Unwrap:

{
    "error": {
        "message": "method not found"
    },
    "traces": [.......]
}

After Unwrap:

{
    "message": "method not found"
}

The structural change above will most likely require changes for the clients of the API.

By using ClearTraces we eliminate the possibility of downstream changes because the structure remains the same:

{
    "error": {
        "message": "method not found"
    }
}

All I want is to simply change the web handlers like this and preserve the structure of the response JSON:

			trace.WriteError(w, trace.ClearTraces(err))

[a-palchikov]

Do we need this in the trace package? So far, this is special-case functionality that works on exported attributes which can be implemented where it is required.
Another question is whether the error types trace provides should be marshaled as they're. Can we consider a way of changing that both ways - e.g. revisit the http marshalling layer that outputs these values in the expected format but obviously without the traces since they don't have them?

[lenko-d]

Closing because the issue will be handled at the marshaling layer.