Rewrite the network TestNet (#3016)

* Add a memory-only option for the network

* Tests cleanup

* Make grandpa/aura/babe compile

* Aura and Babe tests now passing

* More work on tests rewrite

* Attempt to fix grandpa

* Make more grandpa tests pass

* More grandpa tests work

* Work on sync tests

* More work

* light_peer_imports_header_from_announce passes

* can_sync_small_non_best_forks passes

* syncing_node_not_major_syncing_when_disconnected passes

* blocks_are_not_announced_by_light_nodes passing

* All sync tests passing 🎉

* Some TestNet cleanup

* Work on grandpa tests

* More grandpa work

* GrandPa work

* Add check about block_import

* Remove the temporarily added Sync

* Fix network tests warnings

* voter_persists_its_votes passes

* Fix imports in network tests

* Fix service tests

* Call on_block_imported 🤷

* Add shortcut

* Finish using shortcut