Implement erasePolicies() in Storage

Implement erasePolicies() in Storage

Implementation add StorageBackend::erasePolicies() in base class and
its implementation in InMemoryStorageBackend.
In Storage class erasePolicies() just passes this request to backend.

PolicyBucket class was enhanced with getSubBuckets() method returning
set of all IDs of subbuckets. Subbucket is a bucket that can be reached
with policy (type==BUCKET) from current bucket.

Change-Id: I90598aa916857a917d911068da1a1c18c69391a5