Add cookie & crumb to requests. Involves several changes:
- fetch cookie & crumb, obviously.
- two different cookie strategies - one seems to work better in USA, other better outside.
- yfinance auto-detects if one strategy fails, and switches to other strategy.
- cookie is stored in persistent cache folder, alongside timezones. Refetched after 24 hours.

To have this work well with multithreading (yfinance.download()) requires more changes:
- all threads share the same cookie, therefore the same session object. Requires thread-safety ...
- converted data class to a singleton with "SingletonMeta":
 - the first init() call initialises data.
 - but successive calls update its session object - naughty but necessary.
- thread locks to avoid deadlocks and race conditions.