Introduce Storage Manager and Storage Groups feature.

Introduce Storage Manager and Storage Groups feature.

Description:
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1. All of the director's storage-related fields were moved from the
   jcr struct into the new Storage Manager class.
   From now one all of the storage operations (get, set, ...) should be performed
   using manager.
2. Introduced Storage Groups along with different policies (for now only
   the default one and LeastUsed). Storage now can be defined as a list
   inside the Job or Pool resource, along with its policy.

Big part of this commit is just an refactoring (moving fields from jcr
to the class members, adding some helper methods).