Make current Filesystem a Cloud-based one and add new async one

[khelle]

At first Filesystem was added only for the need of reading and writing configuration files. Then, the additional abstracitons were added, because they were easy to implement on top of league's filesystem. There is set of drawbacks of using this apporach and would be beneficial to reimplement it. The proposal is:

• Move current Filesystem (Kraken\Filesystem) to Cloud (Kraken\Cloud) namespace.
• Implement async Filesystem in empty (Kraken\Filesystem).