Replace Reflection with Expression in IdIsEmpty, and some minor changes

Fixed with ConcurrentDictionary.
Thank you for the feedback.

------
Hi, great job with the changes.
I was giving up hope on this ejdb, before I found yours.

My changes consist of:
Changing the IdIsEmpty in TypeExtension to use Expression instead of slow Reflection
Rename FinOne to FindOne in Query class
Reusing of the json serializer 
Some unimportant code reformatting

Thank you.