Merge Protocol and Notifications

[altonen]

After paritytech/substrate#12828, Protocol has no role in the networking stack anymore as the syncing protocol now lives above sc-network and the low-level API used for sync is no longer used. Notification-related events can be sent directly from Notifications.

Tasks:

• Remove HARDCODED_PEERSET_SYNC and all functionality that references the hardcoded Peerset entry
• Move handshake decoding to protocol handlers [1]
• Add implementations for functions such as add_peers_to_set and disconnect_peer for Notifications
• Remove the block_announce_config from Params in src/config.rs and treat it instead as any other Notifications protocol

[1] "By coincidence", paritytech/substrate#12441 converted the default empty handshake to send the role of the node as the default handshake. This means that each protocol now has the capability of learning the role of remote peer by decoding the handshake, meaning that this logic can be moved out of Protocol and the BlockAnnouncesHandshake decoding can also moved to SyncingEngine.

[altonen]

@melekes This could be a good task to get familiar with the networking code. Some of the tasks can be done as independent PRs

[altonen]

I just realized that we can't trust Roles to be in the handshake because it's not in the protocol specification. Until the Peerset is converted into something that can be shared between protocols easily, solutions to this problem are going to be hackish. What basically needs to happen is that BlockAnnouncesHandshake is decoded and the role is stored somewhere (for example temporarily in NetworkWorker) that can then add it to NotificationStreamOpened when it broadcasts that message to all listeners. This would be a short-term fix in order to get things moving and we would clean it up later when handshakes are properly implemented.