Asynchronous-backing compatible Aura, not plugged in (#2573)

* rough draft of potential parent search

* get things compiling

* fmt

* add new function to all RelayChainInterface implementations

* fix compilation

* set slot and timestamp based on relay parent, prepare for find-parent

* skeleton of new aura logic

* fmt

* introduce a collator module in the Aura crate

* extract different implementations into own modules

* make interface more convenient

* docs and todos for lookahead

* refactor basic collator to use new collator utility

* some more refactoring

* finish most of the control flow for new aura

* introduce backend as parameter

* fix compilation

* fix a couple more TODOs

* add an `announce_block` function to collator service

* announce with barrier

* rename block announcement validator to be more specific

* fmt

* clean up unused import errors

* update references to BlockAnnounceValidator

* rename unstable_reimpl

* add AuraUnincludedSegmentApi

* finish rename

* integrate AuraUnincludedSegmentApi

* add a new block announcement validator for backwards compatibility

* add some naive equivocation defenses

* rustfmt

* clean up remaining TODO [now]s

* fmt

* try to fix inprocess-interface

* actually fix compilation

* ignored -> rejected rephrase

* fix test compilation

* fmt

* clippy

cumulus/client/relay-chain-interface/src/lib.rs

@@ -111,7 +111,7 @@ pub trait RelayChainInterface: Send + Sync {
 	/// Get the hash of the current best block.
 	async fn best_block_hash(&self) -> RelayChainResult<PHash>;
 
-	/// Fetch the block header of a given height
+	/// Fetch the block header of a given hash or height, if it exists.
 	async fn header(&self, block_id: BlockId) -> RelayChainResult<Option<PHeader>>;
 
 	/// Get the hash of the finalized block.