Speedup persistent data

Do not read/write ini-file from disk on each access. Read it once at
startup. And write it periodically and at shutdown.

This should finally fixes #1592: UI hang during moving storage (the issue
is closed by now, but now the UI should not hang at moving at all)

This should significantly improves situation with #1542: UI hangs at
sorting by save_path.
Refactor TorrentPersistentData

1. Move QTimer that saves TorrentPersistentData into TorrentPersistentData
itself.

2. Now if somebody tries to access TorrentPersistentData before it is
created or after it is destroyed, we will get a crash instead of
silently read empty value/write to nowhere.