Add a new virtual public method

virtual void forceSpill() {
    // call forceSpill for all its children
};

in PlanStage class.

Each method that implements spilling will override the forceSpill method to spill all its data to disk before calling forceSpill on if its parent.

virtual void forceSpill() {
    spill();
    PlanStage::forceSpill();
};

If a method does not spill, the default forceSpill implementation will be used.

During this task we should make sure that all classic stages that spill also maintain the following metrics

• bool usedDisk : Set to true when the stage has spilled.
• uint64_t spills : The number of times the stage spilled.
• uint64_t spilledBytes : The size, in bytes, of the memory released with spilling.
• uint64_t spilledDataStorageSize : The size, in bytes, of disk space used for spilling.

We should also verify that they release all memory and disk when closed.

The stages that can spill are:

• BlockHashAggStage
• HashAggStage
• SortStage
• WindowStage
• HashLookupStage
• HashLookupUnwindStage
• UniqueStage
• SpoolEagerProducerStage