[Discover] Fix search source behavior

[timroes]

• Change search source structure: persistent search source as parent + volatile search source as child used for actual querying
• Peristent search source is linked to the saved object so it's saved
• Volatile search source stores things like time range, fields, sorting, highlightAll, version, size
• Persistent search source stores things like index pattern, query, filters
• Reporting uses volatile search source (serializeWithParents)
  • This means reporting needs to handle migration once we change the search source serialization in a breaking manner
  • Either handled internally or moving to saved objects
• In the search embeddable, load the persistent search source from the saved object, then re-create the volatile one in memory (should be a separate helper)
• Make sure getSharingData includes all relevant parts to pass to the reporting plugin

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)