[Backup] Initialize logger with tokio runtime

Summary:
Loggers in rust need to be initialized once. Since we don't really
have a main, I instead snuck it in with initializing the tokio runtime.

Part of https://linear.app/comm/issue/ENG-1717

Test Plan:

• Run backup server
• View logs

Reviewers: varun, tomek, max, karol

Reviewed By: max, karol

Subscribers: ashoat, abosh

Differential Revision: https://phab.comm.dev/D5124