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/runtime/westend/src/lib.rs

@@ -45,12 +45,14 @@ use pallet_session::historical as session_historical;
 use pallet_transaction_payment::{CurrencyAdapter, FeeDetails, RuntimeDispatchInfo};
 use parity_scale_codec::{Decode, Encode, MaxEncodedLen};
 use primitives::{
-	slashing, vstaging::NodeFeatures, AccountId, AccountIndex, Balance, BlockNumber,
-	CandidateEvent, CandidateHash, CommittedCandidateReceipt, CoreState, DisputeState,
-	ExecutorParams, GroupRotationInfo, Hash, Id as ParaId, InboundDownwardMessage,
-	InboundHrmpMessage, Moment, Nonce, OccupiedCoreAssumption, PersistedValidationData,
-	PvfCheckStatement, ScrapedOnChainVotes, SessionInfo, Signature, ValidationCode,
-	ValidationCodeHash, ValidatorId, ValidatorIndex, ValidatorSignature, PARACHAIN_KEY_TYPE_ID,
+	slashing,
+	vstaging::{ApprovalVotingParams, NodeFeatures},
+	AccountId, AccountIndex, Balance, BlockNumber, CandidateEvent, CandidateHash,
+	CommittedCandidateReceipt, CoreState, DisputeState, ExecutorParams, GroupRotationInfo, Hash,
+	Id as ParaId, InboundDownwardMessage, InboundHrmpMessage, Moment, Nonce,
+	OccupiedCoreAssumption, PersistedValidationData, PvfCheckStatement, ScrapedOnChainVotes,
+	SessionInfo, Signature, ValidationCode, ValidationCodeHash, ValidatorId, ValidatorIndex,
+	ValidatorSignature, PARACHAIN_KEY_TYPE_ID,
 };
 use runtime_common::{
 	assigned_slots, auctions, crowdloan,
@@ -1649,6 +1651,7 @@ pub mod migrations {
 			ImOnlinePalletName,
 			<Runtime as frame_system::Config>::DbWeight,
 		>,
+		parachains_configuration::migration::v11::MigrateToV11<Runtime>,
 	);
 }
 
@@ -1789,7 +1792,7 @@ sp_api::impl_runtime_apis! {
 		}
 	}
 
-	#[api_version(9)]
+	#[api_version(10)]
 	impl primitives::runtime_api::ParachainHost<Block> for Runtime {
 		fn validators() -> Vec<ValidatorId> {
 			parachains_runtime_api_impl::validators::<Runtime>()
@@ -1933,6 +1936,10 @@ sp_api::impl_runtime_apis! {
 			parachains_runtime_api_impl::async_backing_params::<Runtime>()
 		}
 
+		fn approval_voting_params() -> ApprovalVotingParams {
+			parachains_staging_runtime_api_impl::approval_voting_params::<Runtime>()
+		}
+
 		fn disabled_validators() -> Vec<ValidatorIndex> {
 			parachains_staging_runtime_api_impl::disabled_validators::<Runtime>()
 		}