Modified: CryptoUtils is not a singleton anymore

This avoids multi threading issues.
Key related methods were moved to the QblKeyFactory singleton class.

As discussed with @Gottox 
Is making these fields final actual part of this commit? 
Furthermore, @roeslpa has already done this here
https://github.com/Qabel/qabel-core/pull/168/files#diff-8a32fe82bcee05020e5021c041912e99R25