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/collation-generation/src/error.rs

@@ -28,6 +28,8 @@ pub enum Error {
 	Util(#[from] polkadot_node_subsystem_util::Error),
 	#[error(transparent)]
 	Erasure(#[from] polkadot_erasure_coding::Error),
+	#[error("Parachain backing state not available in runtime.")]
+	MissingParaBackingState,
 }
 
 pub type Result<T> = std::result::Result<T, Error>;