Return information on indexing status

[floere]

The search engine knows exactly how many pods have been indexed after a restart. Return the percentage of indexed pods to help the UI in determining a rough estimate of pods that would be found when fully indexed (and show the exact number if all are indexed).

• [x] Decide on an attribute name.
• [x] Add top-level attribute.
• [x] Return on the Picky format.
• [x] Update in all specs.

[floere]

Attribute name: indexed_ratio: [0.0-1.0]

[segiddins]

indexing_progress ?

[floere]

Good idea!