collation-generation + collator-protocol: collate on multiple assigned cores (#3795)

This works only for collators that implement the `collator_fn` allowing
`collation-generation` subsystem to pull collations triggered on new
heads.

Also enables
`request_v2::CollationFetchingResponse::CollationWithParentHeadData` for
test adder/undying collators.

TODO:
- [x] fix tests
- [x] new tests
- [x] PR doc

---------

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

polkadot/node/subsystem-util/src/lib.rs

@@ -30,7 +30,7 @@ use polkadot_node_subsystem::{
 	messages::{RuntimeApiMessage, RuntimeApiRequest, RuntimeApiSender},
 	overseer, SubsystemSender,
 };
-use polkadot_primitives::{slashing, CoreIndex, ExecutorParams};
+use polkadot_primitives::{async_backing::BackingState, slashing, CoreIndex, ExecutorParams};
 
 pub use overseer::{
 	gen::{OrchestraError as OverseerError, Timeout},
@@ -308,6 +308,7 @@ specialize_requests! {
 	fn request_disabled_validators() -> Vec<ValidatorIndex>; DisabledValidators;
 	fn request_async_backing_params() -> AsyncBackingParams; AsyncBackingParams;
 	fn request_claim_queue() -> BTreeMap<CoreIndex, VecDeque<ParaId>>; ClaimQueue;
+	fn request_para_backing_state(para_id: ParaId) -> Option<BackingState>; ParaBackingState;
 }
 
 /// Requests executor parameters from the runtime effective at given relay-parent. First obtains