[keyserver] Update thread-fetchers to handle image avatars

Summary: Update the queries in thread-fetchers to fetch upload_id and upload_secret from uploads table to create upload URL and include in ClientAvatar.

Test Plan:
Set breakpoints after each query and observed result set and that the ClientAvatars were constructed correctly.

Verified that I was able to see image avatars on native client:
[will add screenshots shortly]

Reviewers: ginsu, ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7393