Provide streaming HTTP API for checking task status

[mgrabovsky]

Currently, Retrace Server provides an HTTP endpoint for checking the status of a task by polling in regular intervals (as used by abrt-retrace-client). It would be nice to provide a streaming endpoint for a smoother user experience, either via WebSockets, EventSource or a similar HTTP-based technology.

[msrb]

I'd probably vote for server-sent events, but I do not have any practical experience with these technologies 👍