Tracing on client can sometimes lose "sendMutations" events.

In TabletServerBatchWriter.MutationWriter.queueMutations(...) (line 736), HTrace Span's are not properly propagated across thread boundaries. Consequently, tracing doesn't propagate into TabletServerBatchWriter.SendTask.send(...), and so the sendMutation event can fail to be logged.

This can cause ShellServerIT.trace(...) to fail.