[services] Backup - client reactors - distinguish terminated from done

Summary:
Depends on D3664

We need to separate two pieces of information: when the reactor is terminated and when it is "done".

• terminated - this means that there are no more operations defined by the user
• done - this means that there will be no more gRPC operations performed for this reactor (all of 'em are done)

Thing is, gRPC does some finalizing operations after we terminate a reactor.

Test Plan:

cd services
yarn run-backup-service

Reviewers: palys-swm

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3719