Fix potential deadlock in SchedulingQueue

[TaoYang526]

There may be a potential deadlock when:
(1) scheduling thread holds the lock of parent queue and waits for read lock of child queue (calling stack: Scheduler#singleStepSchedule -> Scheduler#findAllocationAsks -> Scheduler#findNextAllocationAskCandidate -> Scheduler#sortSubqueuesFromQueue -> SchedulingQueue#GetPendingResource)
and meanwhile
(2) scheduler event handler holds the lock of child queue and waits for read lock of parent queue (calling stack: Scheduler#handleSchedulerEvent -> Scheduler#processAllocationUpdateEvent -> Scheduler#updateSchedulingRequest -> SchedulingQueue#IncPendingResource)

I think the proper sequence of nested lock should be from parent queue to child queue, the contrary way should be corrected.

Deadlock stacks:

[yangwwei]

Nice catch @TaoYang526
I will get this merged once we received your ICLA. Thanks!

[yangwwei]

LGTM

[yangwwei]

Thanks @TaoYang526 for getting this fixed!

[TaoYang526]

Thanks @yangwwei for the review and commit.