client/network-gossip: Integrate GossipEngine tasks into Future impl (#4767)

`GossipEngine` spawns two tasks, one for a periodic tick, one to forward
messages from the network to subscribers. These tasks hold an `Arc` to a
`GossipEngineInner`.

To reduce the amount of shared ownership (locking) this patch integrates
the two tasks into a `Future` implementation on the `GossipEngine`
struct. This `Future` implementation can now be called from a single
owner, e.g. the `finality-grandpa` `NetworkBridge`.

As a side effect this removes the requirement on the `network-gossip`
crate to spawn tasks and thereby removes the requirement on the
`finality-grandpa` crate to spawn any tasks.

This is part of a greater effort to reduce the number of owners of
components within `finality-grandpa`, `network` and `network-gossip` as
well as to reduce the amount of unbounded channels. For details see
d9837d7dd, 5f80929dc and 597c0a6c4.

substrate/client/network-gossip/src/state_machine.rs

@@ -35,6 +35,8 @@ const KNOWN_MESSAGES_CACHE_SIZE: usize = 4096;
 
 const REBROADCAST_INTERVAL: time::Duration = time::Duration::from_secs(30);
 
+pub(crate) const PERIODIC_MAINTENANCE_INTERVAL: time::Duration = time::Duration::from_millis(1100);
+
 mod rep {
 	use sc_network::ReputationChange as Rep;
 	/// Reputation change when a peer sends us a gossip message that we didn't know about.