Unsafe concurrent access to unknown lease collection in AssignableVMs class

[tbak]

unknownLeaseIdsToExpire array is modified directly when invoking TaskScheduler#expireLease method. This array is not concurrent, and most likely the intent was to handle it like all the other mutations by the internal thread.

Here is an example stack trace:
fenzo.TaskScheduler:? - Error with scheduling run: null java.util.ConcurrentModificationException at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901) at java.util.ArrayList$Itr.next(ArrayList.java:851) at com.netflix.fenzo.AssignableVMs.expireAnyUnknownLeaseIds(AssignableVMs.java:257) at com.netflix.fenzo.AssignableVMs.prepareAndGetOrderedVMs(AssignableVMs.java:270) at com.netflix.fenzo.TaskScheduler.doSchedule(TaskScheduler.java:754) at com.netflix.fenzo.TaskScheduler.doScheduling(TaskScheduler.java:736) at com.netflix.fenzo.TaskScheduler.scheduleOnce(TaskScheduler.java:711) at com.netflix.fenzo.TaskSchedulingService.scheduleOnce(TaskSchedulingService.java:275) at com.netflix.fenzo.TaskSchedulingService.access$700(TaskSchedulingService.java:73) at com.netflix.fenzo.TaskSchedulingService$1.run(TaskSchedulingService.java:140) at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308) at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180) at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748)