Existance of node/rel is checked before deletion

When node/relationship is deleted twice in the same transaction, than second delete call throws IllegalStateException (which is ok) and whole transaction is marked as rollback-only (which is too harsh).
This PR adds node/relationship existence check before deletion. This check examines transaction state and falls back to asking disk layer if entity exists.
Instead of creating an unrelated node, isn't it better to check that the relationship has been deleted?