QtHelp: Allow canceling indexing while the index database is locked

The indexer retried to initialize the index database every second for
as long as it stayed locked, without checking the cancel flag. As the
destructor of QHelpSearchIndexWriter waits for the thread to finish,
a persistently locked database hung the application on shutdown.

Check the cancel flag before each retry.

Task-number: QTBUG-150684
Pick-to: 6.12 6.8
Change-Id: I318379e6bd2dd658cbde07663035b3207c5ab341
Reviewed-by: Kai Köhne <kai.koehne@qt.io>