Add zip support to UniqueOnlyQueue. Needed to support fairness across datasources.

[srikalyan chandrashekar]

Zip 'k' lists to form a grand list. For Ex: [[x1, x2, x3] [y1, y2] [z1] [d1, d2, d3]] -> [x1, y1, z1, d1, x2, y2, d2, x3, d3]
* Assuming each list is per datasource and are sorted by id(latest on top) then
* By zipping we are picking one latest from each data source.