two failed tests in Jenkins HBase-TRUNK-on-Hadoop-23 build

[Ted Yu]

I saw the following in
https://builds.apache.org/view/G-L/view/HBase/job/HBase-TRUNK-on-Hadoop-23/lastCompletedBuild/testReport/org.apache.hadoop.hbase.regionserver/TestRpcMetrics/testCustomMetrics/
:

2012-03-09 00:17:59,871 INFO  [pool-1-thread-1]
hfile.CacheConfig(345): Allocating LruBlockCache with maximum size
422.2m
java.util.ConcurrentModificationException       at
java.util.HashMap$HashIterator.nextEntry(HashMap.java:793)
        at java.util.HashMap$ValueIterator.next(HashMap.java:822)
        at 
org.apache.hadoop.hbase.ipc.HBaseRpcMetrics.doUpdates(HBaseRpcMetrics.java:208)
        at 
org.apache.hadoop.metrics.spi.AbstractMetricsContext.timerEvent(AbstractMetricsContext.java:299)
        at 
org.apache.hadoop.metrics.spi.AbstractMetricsContext.access$000(AbstractMetricsContext.java:57)
        at 
org.apache.hadoop.metrics.spi.AbstractMetricsContext$1.run(AbstractMetricsContext.java:264)
        at java.util.TimerThread.mainLoop(Timer.java:512)
        at java.util.TimerThread.run(Timer.java:462)

FYI