Fix CPU high problem when the zipkin collector is down

I think this should be a typo. When the background thread fails to write spans to zipkin collector, it will run into a busy loop.

"zipkinSpanReceiver-0" daemon prio=10 tid=0x00002aaad408b000 nid=0xf08 runnable [0x0000000043766000]
   java.lang.Thread.State: RUNNABLE
        at java.util.ArrayList.size(ArrayList.java:177)
        at java.util.AbstractList$Itr.hasNext(AbstractList.java:339)
        at com.twitter.zipkin.gen.Span$SpanStandardScheme.write(Span.java:935)
        at com.twitter.zipkin.gen.Span$SpanStandardScheme.write(Span.java:798)
        at com.twitter.zipkin.gen.Span.write(Span.java:714)
        at org.cloudera.htrace.impl.ZipkinSpanReceiver$WriteSpanRunnable.run(ZipkinSpanReceiver.java:263)
        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441)
        at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
        at java.util.concurrent.FutureTask.run(FutureTask.java:138)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
        at java.lang.Thread.run(Thread.java:662)