Solving concurency issues when applying configuration

[ljupcovangelski]

When we apply configuration from the CLI, we should be able to detect if the configuration has changed since the last retrieval by the client (either on the CLI or in the frontend ui).

To solve this, proably best approach would be that the controller sends back a checksum of the configuration on GET http://airy.core/controller/config that we can store on the client side. So when we apply the configuration, we would need to also send the last checksum and the controller would verify that there haven't been any changes since the last retrieval.

Related to #2756 .