Allow a gate to accept payloads for several services

Summary:
To accept notifications in native format, plan is to have one gate.
This gate will so receive payloads from different services, and this
service will be available in the payload (in the service property).

Currently, service name is hardcoded as a SERVICE_NAME constant.
Instead, each gate will provide a custom getServiceName() method.

Gates for one service (ie all our current gates) now have such methods
to return the same string than the former SERVICE_NAME value.

More complex gates will customize this logic.

Ref T979

Test Plan: Tested with success with D871 code

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T979

Differential Revision: https://devcentral.nasqueron.org/D897