Make (de)serializer classes more generic and easier testable

Make (de)serializer classes more generic and easier testable

StorageSerializer and StorageDeserializer now accept std::ostream
and std::istream respectively, to enable unit testing.
Storage::insertPolicies() body has been slightly rearranged
to be more readable.

Change-Id: I54296a679a5d1b736ae4f3829f718b6bf8aea66e