HBASE-22634 Improve performance of BufferedMutator

Corrected unit test when neither hbase.htable.threads.max nor
hbase.client.max.total.tasks are set

hbase-client/src/main/java/org/apache/hadoop/hbase/client/BufferedMutatorImpl.java

@@ -63,6 +63,8 @@
  * @see Connection
  * @since 1.0.0
  */
+@edu.umd.cs.findbugs.annotations.SuppressWarnings(value="IS2_INCONSISTENT_SYNC",
+    justification="writeBufferPeriodicFlushTimer needs to be synchronized only when not null, and in mutual exclusion with close() and flush(true). However to needs to synchronize with flush(false) coming from mutate")
 @InterfaceAudience.Private
 @InterfaceStability.Evolving
 public class BufferedMutatorImpl implements BufferedMutator {
@@ -304,7 +306,7 @@ public void flush() throws InterruptedIOException, RetriesExhaustedWithDetailsEx
     boolean haveLocked = false;
     if (writeBufferPeriodicFlushTimer != null) {
       lock.lock();
-      haveLocked = false; // make sure to unlock even if writeBufferPeriodicFlushTimer is set to null before the end
+      haveLocked = true; // make sure to unlock even if writeBufferPeriodicFlushTimer is set to null before the end
     }
     try {
       checkClose();