[services] Lib - Move reactor utility

Summary:
Depends on D3910

Moving reactor utility from both blob/backup to the mutual "lib" space.

Test Plan:
No real changes are applied, just moving stuff around, so services should still build and that's the test plan

cd services

yarn run-blob-service
yarn run-backup-service

Reviewers: palys-swm, ashoat

Reviewed By: ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3911