[lib] Introduce getUploadIDsFromMediaMessageServerDBContents(...)

Summary:
Introduce getUploadIDsFromMediaMessageServerDBContents(...) and consume in fetchUploadsForMessage(...) and assignMessageContainerToMedia(...).

Simple refactor to reduce redundant code.

Addresses feedback from @ashoat here: https://phab.comm.dev/D5087#inline-33653

Test Plan: Simple refactor, things continue to work as expected when sending media messages. Set breakpoints and ensured that values at each step were as expected.

Reviewers: abosh, tomek, marcin, varun

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5190