[CommCoreModule] Introduce createMessageStoreOperations and use in processMessageStoreOperations

Summary: We pull out the part of processMessageStoreOperations that constructs that std::vector of operations so we can also use it in processMessageStoreOperationsSync which will be introduced in the next diff.

Test Plan: SQLite store continues to looks as expected

Reviewers: palys-swm, def-au1t, ashoat

Reviewed By: palys-swm, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D2595