[lib] New action to fetch single message for a list of threads

Summary: Needed to fetch messages for message previews

Test Plan: @def-au1t promised to help test this :)

Reviewers: def-au1t

Reviewed By: def-au1t

Subscribers: palys-swm, Adrian, atul, karol-bisztyga, benschac, yayabosh, def-au1t

Differential Revision: https://phabricator.ashoat.com/D3566