Race condition in SemaphoreCompletionService.executeFront()

If two threads call executeFront() at the same time, it's possible that none of them executes any task, although there are permits available, and one of the threads just added a task in the queue. This scenario causes random initial state transfer timeouts in the testsuite:

1. T1: continueTaskInBackground(): queue = empty, permits = 0
2. T1: backgroundTaskFinished()
3. T1: = semaphore.release(): queue = empty, permits = 1
4. T1: = executeFront()
5. T1: == semaphore.acquire() -> true: queue = empty, permits = 0
6. T2: == queue.poll() -> null
7. T2: submit(task)
8. T2: = executeFront()
9. T2: == semaphore.acquire() -> false
10. T2: return without executing any task
11. T1: return without executing any task

Failure example: http://ci.infinispan.org/viewLog.html?buildId=27153&tab=buildResultsDiv&buildTypeId=Infinispan_MasterHotspotJdk8#testNameId3522947143330444111