**Interface changes**
There are two goals behind the interface changes:
* Make it clearer which courier modules are meant for public consumption by exporting them from the top level.
* Simplify the courier object by removing responsibilities and focusing its responsibility solely on scheduling search requests via the fetch method and timefilter.refreshInterval Angular event.
I did this by taking the following steps:
* Removing redirectWhenMissing, indexPatterns, SearchSource, and SavedObject from the courier object. I also removed some unused methods from its interface.
* redirectWhenMissing is now a service registered on the kibana/url Angular module.
* indexPatterns is now a service registered on the kibana/index_patterns Angular module.
* SearchSourceProvider and SavedObjectProvider are now top-level exports of ui/courier.
* migrateFilter, decorateQuery, buildQueryFromFilters, and luceneStringToDsl are now top-level exports of ui/courier.
**Internal changes**
I also made some internal changes in an effort to organize the code clearly and reduce unnecessary complexity.
* I refactored the async code in CallClient to appear sync with async/await and encapsulated chunks of logic in helper functions. I also used an isAborted flag instead of overwriting the esPromise var with an enum.
* I combined Looper and SearchLooper into a single class and deleted unused functions.
* I reorganized the courier/fetch/request code into SearchRequest, SegmentedSearchRequest, and serializeFetchParams modules.
* Renamed various other methods and variables to improve clarity.
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