[server] Rename ThreadSelectionCriteria to MessageSelectionCriteria

Summary: This type is used for filtering the results of fetchMessageInfos and fetchMessageInfosSince. Up until now, ThreadSelectionCriteria only filtered by thread. In the next diff, I will be adding a new way of filtering that has nothing to do with threads.

Test Plan: Flow (this is just a rename)

Reviewers: palys-swm, atul

Reviewed By: palys-swm, atul

Subscribers: benschac, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3347