[keyserver] Pull fetchUploadsForMessage(...) out of fetchMediaFromMediaMessageContent(...)

Summary:
Pull the serverDB uploads table query out of fetchMediaFromMediaMessageContent(...) into a separate function to make things a bit shorter and more readable.

This is effectively a noop, just repositioning things.

Depends on D5085

Test Plan: NA, effectively a noop...but sent a few messages and checked values at various breakpoints as a sanity check to ensure things worked as expected.

Reviewers: abosh, marcin, tomek

Reviewed By: tomek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5086