[lib] Correctly update server thread id when selecting users

Test Plan:
Search for a pending thread with at least 2 users. Send a message to the thread and check if it was created.
OPen pending sidebar, send a message and check if it was created.

Reviewers: ashoat, KatPo

Reviewed By: ashoat, KatPo

Subscribers: KatPo, zrebcu411, Adrian, atul, subnub, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D756