Remove unwanted pointers (InMemoryStorageBackend)

Remove unwanted pointers (InMemoryStorageBackend)

Using pointers to helper classes (Integrity and, in future,
ChecksumValidator) is unnecessary and undesired. However, this forces
removal of InMemoryStorageBackend default constructor, as it cannot
initialize its helpers with valid arguments.

This patch also adjusts tests to the new constructor set and removes no
longer used typedef.

Change-Id: If0a41a75c16be6d55bdf3841ddb5190c388968af