Push service tokens cleanup #399
Description
An FCM token is unique for a device. We connect the FCM token to an address and then send notifications to that fcm token.
In some situations it is possible to register the same FCM token to a different address. This would mean that a user could get push notifications for an account he is currently logged out from.
An easy fix on the relay server would be to cleanup the old addresses with the same FCM token, whenever someone tries to register a new address with the same FCM token.