Approve multiple candidates with a single signature (#1191)

Initial implementation for the plan discussed here: https://github.com/paritytech/polkadot-sdk/issues/701
Built on top of https://github.com/paritytech/polkadot-sdk/pull/1178
v0: https://github.com/paritytech/polkadot/pull/7554,

## Overall idea

When approval-voting checks a candidate and is ready to advertise the
approval, defer it in a per-relay chain block until we either have
MAX_APPROVAL_COALESCE_COUNT candidates to sign or a candidate has stayed
MAX_APPROVALS_COALESCE_TICKS in the queue, in both cases we sign what
candidates we have available.

This should allow us to reduce the number of approvals messages we have
to create/send/verify. The parameters are configurable, so we should
find some values that balance:

- Security of the network: Delaying broadcasting of an approval
shouldn't but the finality at risk and to make sure that never happens
we won't delay sending a vote if we are past 2/3 from the no-show time.
- Scalability of the network: MAX_APPROVAL_COALESCE_COUNT = 1 &
MAX_APPROVALS_COALESCE_TICKS =0, is what we have now and we know from
the measurements we did on versi, it bottlenecks
approval-distribution/approval-voting when increase significantly the
number of validators and parachains
- Block storage: In case of disputes we have to import this votes on
chain and that increase the necessary storage with
MAX_APPROVAL_COALESCE_COUNT * CandidateHash per vote. Given that
disputes are not the normal way of the network functioning and we will
limit MAX_APPROVAL_COALESCE_COUNT in the single digits numbers, this
should be good enough. Alternatively, we could try to create a better
way to store this on-chain through indirection, if that's needed.

## Other fixes:
- Fixed the fact that we were sending random assignments to
non-validators, that was wrong because those won't do anything with it
and they won't gossip it either because they do not have a grid topology
set, so we would waste the random assignments.
- Added metrics to be able to debug potential no-shows and
mis-processing of approvals/assignments.

## TODO:
- [x] Get feedback, that this is moving in the right direction. @ordian
@sandreim @eskimor @burdges, let me know what you think.
- [x] More and more testing.
- [x]  Test in versi.
- [x] Make MAX_APPROVAL_COALESCE_COUNT &
MAX_APPROVAL_COALESCE_WAIT_MILLIS a parachain host configuration.
- [x] Make sure the backwards compatibility works correctly
- [x] Make sure this direction is compatible with other streams of work:
https://github.com/paritytech/polkadot-sdk/issues/635 &
https://github.com/paritytech/polkadot-sdk/issues/742
- [x] Final versi burn-in before merging

---------

Signed-off-by: Alexandru Gheorghe <alexandru.gheorghe@parity.io>

polkadot/node/network/protocol/src/grid_topology.rs

@@ -73,12 +73,20 @@ pub struct SessionGridTopology {
 	shuffled_indices: Vec<usize>,
 	/// The canonical shuffling of validators for the session.
 	canonical_shuffling: Vec<TopologyPeerInfo>,
+	/// The list of peer-ids in an efficient way to search.
+	peer_ids: HashSet<PeerId>,
 }
 
 impl SessionGridTopology {
 	/// Create a new session grid topology.
 	pub fn new(shuffled_indices: Vec<usize>, canonical_shuffling: Vec<TopologyPeerInfo>) -> Self {
-		SessionGridTopology { shuffled_indices, canonical_shuffling }
+		let mut peer_ids = HashSet::new();
+		for peer_info in canonical_shuffling.iter() {
+			for peer_id in peer_info.peer_ids.iter() {
+				peer_ids.insert(*peer_id);
+			}
+		}
+		SessionGridTopology { shuffled_indices, canonical_shuffling, peer_ids }
 	}
 
 	/// Produces the outgoing routing logic for a particular peer.
@@ -111,6 +119,11 @@ impl SessionGridTopology {
 
 		Some(grid_subset)
 	}
+
+	/// Tells if a given peer id is validator in a session
+	pub fn is_validator(&self, peer: &PeerId) -> bool {
+		self.peer_ids.contains(peer)
+	}
 }
 
 struct MatrixNeighbors<R, C> {
@@ -273,6 +286,11 @@ impl SessionGridTopologyEntry {
 	pub fn get(&self) -> &SessionGridTopology {
 		&self.topology
 	}
+
+	/// Tells if a given peer id is validator in a session
+	pub fn is_validator(&self, peer: &PeerId) -> bool {
+		self.topology.is_validator(peer)
+	}
 }
 
 /// A set of topologies indexed by session
@@ -347,6 +365,7 @@ impl Default for SessionBoundGridTopologyStorage {
 					topology: SessionGridTopology {
 						shuffled_indices: Vec::new(),
 						canonical_shuffling: Vec::new(),
+						peer_ids: Default::default(),
 					},
 					local_neighbors: GridNeighbors::empty(),
 				},