[keyserver] Add index to container column of uploads table

Summary: This is the first part of the stack to resolve ENG-1484. We'll start using it in message-fetchers.js in the following diff.

Test Plan: I first tested this stack locally in my environment to make sure the queries worked. Then I patched it onto my production keyserver and confirmed performance was improved

Reviewers: tomek, atul

Reviewed By: tomek

Subscribers: adrian, abosh

Differential Revision: https://phab.comm.dev/D4673