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/bridge/src/network.rs

@@ -33,7 +33,7 @@ use sc_network::{
 use polkadot_node_network_protocol::{
 	peer_set::{CollationVersion, PeerSet, ProtocolVersion, ValidationVersion},
 	request_response::{OutgoingRequest, Recipient, ReqProtocolNames, Requests},
-	v1 as protocol_v1, v2 as protocol_v2, vstaging as protocol_vstaging, PeerId,
+	v1 as protocol_v1, v2 as protocol_v2, v3 as protocol_v3, PeerId,
 };
 use polkadot_primitives::{AuthorityDiscoveryId, Block, Hash};
 
@@ -62,20 +62,20 @@ pub(crate) fn send_validation_message_v1(
 	);
 }
 
-// Helper function to send a validation vstaging message to a list of peers.
+// Helper function to send a validation v3 message to a list of peers.
 // Messages are always sent via the main protocol, even legacy protocol messages.
-pub(crate) fn send_validation_message_vstaging(
+pub(crate) fn send_validation_message_v3(
 	peers: Vec<PeerId>,
-	message: WireMessage<protocol_vstaging::ValidationProtocol>,
+	message: WireMessage<protocol_v3::ValidationProtocol>,
 	metrics: &Metrics,
 	notification_sinks: &Arc<Mutex<HashMap<(PeerSet, PeerId), Box<dyn MessageSink>>>>,
 ) {
-	gum::trace!(target: LOG_TARGET, ?peers, ?message, "Sending validation vstaging message to peers",);
+	gum::trace!(target: LOG_TARGET, ?peers, ?message, "Sending validation v3 message to peers",);
 
 	send_message(
 		peers,
 		PeerSet::Validation,
-		ValidationVersion::VStaging.into(),
+		ValidationVersion::V3.into(),
 		message,
 		metrics,
 		notification_sinks,