search refactoring

[sbwalker]

• removed ISearchIndexManager extensibility (ISearchable should be the primary extensibility option)
• removed methods with delegate parameters as they have poor encapsulation, have too much indirection, are difficult to understand and debug
• fixed issue setting Alias when indexing
• modified SearchIndexJob to handle pages, modules which implement ISearchable and modules which do not implement ISearchable
• added support for reindexing when a parent object (ie. page or module) changes
• indexed modules which appear on multiple pages (ie. unique urls)
• improved performance by using PageModule.Page rather than calling GetPage()
• made scheduled job, ISearchable interface, and SearchProvider methods async
• ensured deleted content is purged from SearchContent
• optimized ISearchable implementation in HtmlText module
• replaced Utility IsPageModuleVisible with IsEffectiveOrExpired
• created constant InternalPagePaths which is used by search indexer and site map generator