Implement new API to retrieve the health status

[sashafrey]

Introduce a new method in c_interface --- ArtmGetHealthStatus(), which outputs a protobuf message with several diagnostic field, including

• the percent of completion during the iteration (for example, the percent of processed batches)
• the status of processor and merger queues
• connection status (connected / disconnected) of all remote nodes (if in network modus operandi)
• memory usage statistics (topic models, merger queue, data loader queue, theta-cache in data loader, etc)
• etc (other relevant information that might be helpful in diagnostics)