[server] Prevent threads from being converted to SIDEBARs

Summary: If you're not born a SIDEBAR, you can't become one.

Test Plan: careful reading

Reviewers: palys-swm, KatPo

Reviewed By: palys-swm, KatPo

Subscribers: zrebcu411, Adrian, atul, subnub

Differential Revision: https://phabricator.ashoat.com/D952