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.
Is there a better way to check that relationship is deleted than combination of: `db.getRelationshipById` & `expected = NotFoundException` ?