Implement Storage::save() using StorageSerializer and clean up

Implement Storage::save() using StorageSerializer and clean up

There was a lot of mess in #includes directives and forward
declarations of classes. It was cleaned out.
InMemoryStorageBackend::Buckets were moved to separate file
in order not to make a cyclic dependency.

New exception class CannotCreateFile was added.

Change-Id: I32b234ba0f4872791a9f931690c58e623de7096f