Async rendering of change logs

[dcoraboeuf]

Rendering long change logs can be very costly in time (because of back and forth calls to the backend ticketing systems for example), leading to timeouts and no change log being rendered.

For each API call which renders change logs (like Build.render, Build.gitChangeLog - deprecated - or scmChangeLog), add an async alternative.

The call just returns the ID of the change log and the rendering happens in the background. The client can check for the status of the change log at regular intervals.

The computed change logs are stored in Ontrack, and some settings are available to ensure some regular cleanup.