V3 search and autocomplete throw NullReferenceException on index swap

[maartenba]

This PR fixes NuGet/NuGetGallery#2520:

• Adds support for SearcherManager-specific IndexSearcher implementation
• Updates NuGetSearcherManager to use a NuGetIndexSearcher
• Adds all lookups and auxiliary data that is specific to the loaded index on NuGetIndexSearcher instead of NuGetSearcherManager
• Removes a lock while loading auxiliary data (can now be safely done as it is part of the NuGetIndexSearcher)
• Reduces memory usage for auxiliary data (e.g. versions by document lookup) as it no longer stores all data for bot http and https scheme

@bhuvak @johnataylor @xavierdecoster Can you review code changes?

[johnataylor]