[Backup] Use only one tokio runtime

Summary:
We only need one runtime to do asynchronous calls. Using two or more
is an anti-pattern.

https://linear.app/comm/issue/ENG-1808
https://linear.app/comm/issue/ENG-1806

Test Plan:

cd services/backup
rm -rf build && mkdir build && cd build && cmake .. && make -j

Reviewers: atul, varun, karol, max

Reviewed By: karol, max

Subscribers: ashoat, tomek, abosh

Differential Revision: https://phab.comm.dev/D5153