[services] Backup - return log id of the newly created log to the client

Summary:
Depends on D4033

In order to Be able to send attachments for certain logs, we should first get new logs ids from the backup service.

Test Plan:

• terminal 1

cd services
yarn run-blob-service-dev-mode

• terminal 2

cd services
yarn run-backup-service-dev-mode

• terminal 3

git clone https://github.com/karol-bisztyga/grpc-playground.git
cd grpc-playground
git checkout backup-async
./build.sh
./cmake/build/bin/client

Then in terminal 3 create a new backup

n

Then in terminal 3 create a new log

l

And see that you obtained a new log id.

Reviewers: palys-swm, ashoat

Reviewed By: palys-swm, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D4074