Fetch overlapping shards in parallel

Summary:
The gains are pretty minimal because we only have 3 shards per bucket. The more shards we have the more this will outperform the older solution. We should probably crank up number of shards and increase the time_width.

http://datastax.github.io/python-driver/performance.html

Test Plan: -

Reviewers: marcua

Reviewed By: marcua

Differential Revision: https://phabricator.locu.com/D7123