Forks support in sync (#91)

* intersect_with_inventory

* process_new_blocks_inventory fixed to support forks

* fixed sync issues

* fixed on_peer_block to support forks

* forks support in on_block_verification_*

* cleaning up

* cleaning up

* forget about best inventory

* ask for inventory after saturating

* '000..000'.into() -> H256::from(0)