[server] Add non-member thread to focused table

Summary:
Focused table didn't include non-member threads - they were filtered out. We need to have them to recieve messages when non-member thread is opened and watched.
Instead of filtering non-members at the beginning, we will keep them to update focused table, and filter later

Test Plan: Open thread as non-member, send message to this thread and see that message was delivered, check if non-member thread is added to focused table

Reviewers: palys-swm, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, palys-swm, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D1071