[keyserver] Make mediaFromRow return a Promise

Summary:
In D4521 we end up needing to await getDBType() in the message-fetchers.js code because we need to parse the query results differently depending on whether it came from MySQL 5.7 or MariaDB 10.8.

Depends on D4519

Test Plan: Flow; tested in combination with next diffs

Reviewers: jonringer-comm, atul, palys-swm

Reviewed By: atul

Subscribers: Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4520