Have you considered http://mqtt.org/? Its a lightweight protocol with a publish/subscribe model that might fit your application nicely. List of public brokers at public_brokers · mqtt/mqtt.org Wiki · GitHub
Otherwise perhaps a redis or mongodb hosted database? For example https://redislabs.com/pricing/cloud/ (free for 30Mb of data and 30 connections) or MongoDB Atlas Database | Multi-Cloud Database Service | MongoDB.