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/statement-distribution/src/lib.rs

@@ -27,7 +27,7 @@ use std::time::Duration;
 
 use polkadot_node_network_protocol::{
 	request_response::{v1 as request_v1, v2::AttestedCandidateRequest, IncomingRequestReceiver},
-	v2 as protocol_v2, vstaging as protocol_vstaging, Versioned,
+	v2 as protocol_v2, v3 as protocol_v3, Versioned,
 };
 use polkadot_node_primitives::StatementWithPVD;
 use polkadot_node_subsystem::{
@@ -400,11 +400,11 @@ impl<R: rand::Rng> StatementDistributionSubsystem<R> {
 							Versioned::V2(
 								protocol_v2::StatementDistributionMessage::V1Compatibility(_),
 							) |
-							Versioned::VStaging(
-								protocol_vstaging::StatementDistributionMessage::V1Compatibility(_),
+							Versioned::V3(
+								protocol_v3::StatementDistributionMessage::V1Compatibility(_),
 							) => VersionTarget::Legacy,
 							Versioned::V1(_) => VersionTarget::Legacy,
-							Versioned::V2(_) | Versioned::VStaging(_) => VersionTarget::Current,
+							Versioned::V2(_) | Versioned::V3(_) => VersionTarget::Current,
 						},
 						_ => VersionTarget::Both,
 					};