Address findbugs/spotbugs complaints (branch-1.4)

FindBugs module:hbase-common
Inconsistent synchronization of org.apache.hadoop.hbase.io.encoding.EncodedDataBlock$BufferGrabbingByteArrayOutputStream.ourBytes; locked 50% of time Unsynchronized access at EncodedDataBlock.java:50% of time Unsynchronized access at EncodedDataBlock.java:[line 258]
FindBugs module:hbase-hadoop2-compat
java.util.concurrent.ScheduledThreadPoolExecutor stored into non-transient field MetricsExecutorImpl$ExecutorSingleton.scheduler At MetricsExecutorImpl.java:MetricsExecutorImpl$ExecutorSingleton.scheduler At MetricsExecutorImpl.java:[line 51]
FindBugs module:hbase-server
Possible null pointer dereference of r in org.apache.hadoop.hbase.regionserver.HStore.removeCompactedfiles(Collection) Dereferenced at HStore.java:r in org.apache.hadoop.hbase.regionserver.HStore.removeCompactedfiles(Collection) Dereferenced at HStore.java:[line 2840]