[lib] Use pending thread id instead of key

Summary: Pending thread key was needed to uniquely identify thread candidates. It's better to use id instead, because the function which creates it can be used in more places.

Test Plan: Create sidebar. Use thread creator and search for users: verify that the correct thread is matched.

Reviewers: palys-swm

Subscribers: KatPo, zrebcu411, Adrian, atul, subnub

Differential Revision: https://phabricator.ashoat.com/D923