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>

cumulus/client/relay-chain-rpc-interface/src/rpc_client.rs

@@ -32,7 +32,7 @@ use cumulus_primitives_core::{
 	relay_chain::{
 		async_backing::{AsyncBackingParams, BackingState},
 		slashing,
-		vstaging::NodeFeatures,
+		vstaging::{ApprovalVotingParams, NodeFeatures},
 		BlockNumber, CandidateCommitments, CandidateEvent, CandidateHash,
 		CommittedCandidateReceipt, CoreState, DisputeState, ExecutorParams, GroupRotationInfo,
 		Hash as RelayHash, Header as RelayHeader, InboundHrmpMessage, OccupiedCoreAssumption,
@@ -625,6 +625,19 @@ impl RelayChainRpcClient {
 	}
 
 	#[allow(missing_docs)]
+	pub async fn parachain_host_staging_approval_voting_params(
+		&self,
+		at: RelayHash,
+		_session_index: SessionIndex,
+	) -> Result<ApprovalVotingParams, RelayChainError> {
+		self.call_remote_runtime_function(
+			"ParachainHost_staging_approval_voting_params",
+			at,
+			None::<()>,
+		)
+		.await
+	}
+
 	pub async fn parachain_host_para_backing_state(
 		&self,
 		at: RelayHash,