extract thread local into scope_stack method

          def start(name, id, payload)

            scope = OpenTracing.start_active_span(span_name(payload))

            stack = Thread.current[:_labkit_trace_scope_stack] ||= []

            stack.push scope

            scope_stack.push scope

          end

          def finish(name, id, payload)

            stack = Thread.current[:_labkit_trace_scope_stack]

            scope = stack.pop

            scope = scope_stack.pop

            span = scope.span

            exception = payload[:exception]

            scope.close

          end

          def scope_stack

            Thread.current[:_labkit_trace_scope_stack] ||= []

          end

          def span_name(payload)

            raise "span_name not implemented"

          end