Fetch larger chunks in KronosClient.get

Summary: With this change, there is no difference in the time it takes to fetch all data with or without `stream=True`. Previously with the default chunk size of 512 bytes we were seeing a 30+% slow down.

Test Plan: -

Reviewers: dyschultz, lydia, marcua

Reviewed By: marcua

Differential Revision: https://phabricator.locu.com/D7079