Improve Distinct operator

Currently Distinct operator is blocking operator. Make it stream operator for following reasons.

• Improve performance
• If we only need to know if the document already present not the document, this reduces memory usage
• We can preserve the order of the input. This allows to use index order.

This will help in the following cases.

• Distinct in projection
• UNION queries
• Part of INTERSECT, INTERSECT ALL, EXCEPT, EXCEPT ALL querires