Revision 650956

synchronize the asyncWrite method. To avoid the following reported stack trace:

Running org.apache.activemq.broker.region.cursors.CursorDurableTest
Exception in thread "Persistence Adaptor Task" java.lang.NullPointerException
        at org.apache.activemq.store.amq.AMQMessageStore$4.execute(AMQMessageStore.java:381)
        at org.apache.activemq.util.TransactionTemplate.run(TransactionTemplate.java:44)
        at org.apache.activemq.store.amq.AMQMessageStore.doAsyncWrite(AMQMessageStore.java:374)
        at org.apache.activemq.store.amq.AMQMessageStore.asyncWrite(AMQMessageStore.java:341)
        at org.apache.activemq.store.amq.AMQMessageStore$1.iterate(AMQMessageStore.java:95)
        at org.apache.activemq.thread.PooledTaskRunner.runTask(PooledTaskRunner.java:122)
        at org.apache.activemq.thread.PooledTaskRunner$1.run(PooledTaskRunner.java:43)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:650)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:675)
        at java.lang.Thread.run(Thread.java:595)
Tests run: 1, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 303.672 sec <<< FAILURE!