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/bin/node-template/node/src/service.rs

@@ -159,7 +159,6 @@ pub fn new_full(config: Configuration<GenesisConfig>)
 				grandpa_link,
 				service.network(),
 				service.on_exit(),
-				service.spawn_task_handle(),
 			)?);
 		},
 		(true, false) => {
@@ -172,7 +171,6 @@ pub fn new_full(config: Configuration<GenesisConfig>)
 				on_exit: service.on_exit(),
 				telemetry_on_connect: Some(service.telemetry_on_connect_stream()),
 				voting_rule: grandpa::VotingRulesBuilder::default().build(),
-				executor: service.spawn_task_handle(),
 			};
 
 			// the GRANDPA voter task is considered infallible, i.e.