Stop getting users if we reach 2000 users. Slack will rate-limit us
even if we follow their limits.
This means that we now have to lookup every user that says a message
for the first time. This should be less intensive on the API.
This also disables partly fb713ed91bfb48c0037e489f80be85c54e69953a for now.
ChannelMembers will not be filled.