[server] Move fetchMessageInfosSince's currentAsOf param to MessageSelectionCriteria

Summary:
Instead of having this param passed in separately, this diff combines it into MessageSelectionCriteria. The reason for this is that we want to allow fetchMessageInfos to filter in this way too.

Depends On D3347

Test Plan: I tested fetchMessageInfosSince (socket connect) and fetchMessageInfos (log in, scrolling to fetch older messages)

Reviewers: palys-swm, atul

Reviewed By: palys-swm

Subscribers: benschac, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3348