Sync times out with 1000 sources

[sssoleileraaa]

Description

Sync times out when there are 1000 sources.

STR

1. run the client (in Qubes preferrably) after you've populated your server with 1000 sources (see https://github.com/freedomofpress/securedrop-client/issues/1007#issuecomment-605218219)
2. reply to a source and send a message as a source

Expected

You should be able to use the client and see new replies and messages succeed.

Actual

The sync times out on a thousand sources, even after a client restart, the sync times out on a 1000 sources.