Create annotations on significant model changes.

[przemekwitek]

In order to give users more insight into anomaly detection, we could be creating an annotation every time there is a significant change in the model calculation. Some ideas on what to report are:

• step change detected
• periodicity detected
• trend detected

How this could work technically:

• CPP emits log messages with special marker string denoting log message related to a significant model change
• Java adds a hook to CppLogMessageHandler class so that every time such message is discovered, an annotation is created
• Annotation is automatically visible in the UI, just like user-created annotations or other system-created annotations (e.g.: model snapshot annotation)
• When job is deleted, we don't need to delete the annotations as they provide history that the user may want to retain

[elasticmachine]

Pinging @elastic/ml-core (:ml)

[droberts195]

Fixed by elastic/ml-cpp#1247