Add cache version value to the journal

[MSDarwish2000]

Is your feature request related to a problem? Please describe.
When the application changes the way it is generating the cached values, or transforming the keys, it will be required to clear the cache and starts a new one.

Describe the solution you'd like
Adding a cache version parameter to the journaled cache and writing it to the journal header will help the library to track these changes and clear the cache directory if the version is changed than the passed one.

Describe alternatives you've considered
Leaving this to application developers to handle but it is easier to handle internally.

Additional context
This behavior is already seen in Jake Wharton's DiskLruCache for example.