[lib] Introduce translateClientDBMessageInfosToRawMessageInfos

Summary: Will be used to create messageStore.messages "in the correct shape" from an array of clientDBMessageInfos

Test Plan: Will be tested with subsequent diffs

Reviewers: palys-swm, def-au1t, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, Adrian, karol-bisztyga, benschac, boristopalov

Differential Revision: https://phabricator.ashoat.com/D2598