Summary:
1. add a test for unsubscription service
2. change the callback to accept const subscription&
3. renamed Send -> SendHelper
4. created a more usable Send
5. changed the Send usages
Reviewers: buda, lion, sasa.stanko
Reviewed By: sasa.stanko
Subscribers: pullbot
Differential Revision: https://phabricator.memgraph.io/D647