Slot leaks if SharedSlotOversubscribedException happens

If a SharedSlotOversubscribedException happens, the MultiTaskSlot will release some of its child SingleTaskSlot. The triggered releasing will trigger a re-allocation of the task slot right inside SingleTaskSlot#release(...). So that a previous allocation in SloSharingManager#allTaskSlots will be replaced by the new allocation because they share the same slotRequestId.
However, the SingleTaskSlot#release(...) will then invoke MultiTaskSlot#releaseChild to release the previous allocation with the slotRequestId, which will unexpectedly remove the new allocation from the SloSharingManager.
In this way, slot leak happens because the pending slot request is not tracked by the SloSharingManager and cannot be released when its payload terminates.

A test case testNoSlotLeakOnSharedSlotOversubscribedException which exhibits this issue can be found in this commit.

The slot leak blocks the TPC-DS queries on flink 1.10, see FLINK-14674.

To solve it, I'd propose to strengthen the MultiTaskSlot#releaseChild to only remove its true child task slot from the SloSharingManager, i.e. add a check if (child == allTaskSlots.get(child.getSlotRequestId())) before invoking allTaskSlots.remove(child.getSlotRequestId()).