Implement healthcheck feature

In order to get an idea of the status of a client, HiveMind Core should be able to send a message at interval *(configurable)* to the client, if the client answer then HiveMind Core consider the client alive and can update the DB *(`last_seen`)*.

The feature should support these criterias:
- Interval between check
- How long to wait for the client response
- How many message not returned before considered "unresponsive"

This configuration should be supported by the `server.json` configuration file.

This is an addition to https://github.com/JarbasHiveMind/HiveMind-core/pull/44