M6IntersectionIterator::GetCount returns the wrong quantity.

[cbaakman]

M6IntersectionIterator's 'mCount' is set in the M6IntersectionIterator::AddIterator function. However, the actual value can not be determined at that time.

To know the correct value, one should loop through all documents in all child iterators. Only documents present in all children should be counted.

One of the symptoms is that the server displays the wrong number of hits when query 'ft:carbohyd AND de:alzheimer' is enetered.