Adding the new search engine abstraction (#13304)

* WIP

* Adding bleve to go modules

* WIP

* Adding missing files from searchengine implementation

* WIP

* WIP

* WIP

* WIP

* WIP

* WIP

* User and channel indexing and searches implemented

* Make bleve tests run with in-memory indexes

* Implement post index and deletion tests

* Initial commits for the search layer

* Removing unnecesary indexing

* WIP

* WIP

* More fixes for tests

* Adding the search layer

* Finishing the migration of searchers to the layer

* Removing unnecesary code

* Allowing multiple engines active at the same time

* WIP

* Add simple post search

* Print information when using bleve

* Adding some debugging to understand better how the searches are working

* Making more dynamic config of search engines

* Add post search basics

* Adding the Purge API endpoint

* Fixing bleve config updates

* Adding missed file

* Regenerating search engine mocks

* Adding missed v5 to modules imports

* fixing i18n

* Fixing some test around search engine

* Removing all bleve traces

* Cleaning up the vendors directory and go.mod/go.sum files

* Regenerating timer layer

* Adding properly the license

* Fixing govet shadow error

* Fixing some tests

* Fixing TestSearchPostsFromUser

* Fixing another test

* Fixing more tests

* Fixing more tests

* Removing SearchEngine redundant text from searchengine module code

* Fixing some reindexing problems in members updates

* Fixing tests

* Addressing PR comments

* Reverting go.mod and go.sum

* Addressing PR comments

* Fixing tests compilation

* Fixing govet

* Adding search engine stop method

* Being more explicit on where we use includeDeleted

* Adding GetSqlSupplier test helper method

* Mocking elasticsearch start function

* Fixing tests

Co-authored-by: Miguel de la Cruz <miguel@mcrx.me>
Co-authored-by: mattermod <mattermod@users.noreply.github.com>

store/sqlstore/user_store.go

@@ -1716,3 +1716,35 @@ func (us SqlUserStore) DemoteUserToGuest(userId string) *model.AppError {
 	}
 	return nil
 }
+
+func (us SqlUserStore) AutocompleteUsersInChannel(teamId, channelId, term string, options *model.UserSearchOptions) (*model.UserAutocompleteInChannel, *model.AppError) {
+	autocomplete := &model.UserAutocompleteInChannel{}
+	uchan := make(chan store.StoreResult, 1)
+	go func() {
+		users, err := us.SearchInChannel(channelId, term, options)
+		uchan <- store.StoreResult{Data: users, Err: err}
+		close(uchan)
+	}()
+
+	nuchan := make(chan store.StoreResult, 1)
+	go func() {
+		users, err := us.SearchNotInChannel(teamId, channelId, term, options)
+		nuchan <- store.StoreResult{Data: users, Err: err}
+		close(nuchan)
+	}()
+
+	result := <-uchan
+	if result.Err != nil {
+		return nil, result.Err
+	}
+	users := result.Data.([]*model.User)
+	autocomplete.InChannel = users
+
+	result = <-nuchan
+	if result.Err != nil {
+		return nil, result.Err
+	}
+	users = result.Data.([]*model.User)
+	autocomplete.OutOfChannel = users
+	return autocomplete, nil
+}