[SqlDatabase] New implementation, hold connections for threads

Instead of using worker thread, use multiple threads with QtConcurrent,
and hold database connections for each thread.
[Crashlogs] Fix showing bad path when saving crashlogs

Regression from recent DataPaths commits
[LocationCompleter] Run completions search in separate thread

Currently, there is one issue with inline domain completion. It is
also searched from the separate thread, and thus it gets displayed
after a small delay.

Closes #1135