[services] Backup - Pull Backup - Wait for the get reactor to terminate

Summary:
Depends on D4326

This is a bug fix which was spotted in https://phab.comm.dev/D4228#121209

The problem was, we didn't wait for the getReactor to gracefully terminate in the PullBackupReactor. The result was, if the client stopped working, the whole service crashed. Now it just shows an error and all objects are deleted properly.

Test Plan: Start pulling the backup and crash the client (you can probably use ctrl+c but I just used panic in rust) in the middle of receiving compaction chunks.

Reviewers: palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4341