It was possible that fetchNextTasks() would be called by multiple
threads concurrently.
If it happens, some tasks in the task queues might be ignored.

cherrypick of Idc81c43c45e382da3850cc55b9a42c281548d2a8

bug: 10957075
Change-Id: I41bf90dda15306ee879350f96a791d940ea31da1