Batch transactions in complex relays (#1669)

* batch transactions in message relay: API prototype

* get rid of Box<dyn BatchTransaction> and actually submit it

* test batch transactions

* message_lane_loop_works_with_batch_transactions

* removed logger

* BatchConfirmationTransaction + BatchDeliveryTransaction

* more prototyping

* fmt

* continue with batch calls

* impl BatchCallBuilder for ()

* BatchDeliveryTransaction impl

* BundledBatchCallBuilder

* proper impl of BundledBatchCallBuilder + use it in RialtoParachain -> Millau

* impl prove_header in OnDemandHeadersRelay

* impl OnDemandParachainsRelay::prove_header (needs extensive tests)

* added a couple of TODOs

* return Result<Option<BatchTx>> when asking for more headers

* prove headers when reauire_* is called && return proper headers from required_header_id

* split parachains::prove_header and test select_headers_to_prove

* more traces and leave TODOs

* use finality stream in SubstrateFinalitySource::prove_block_finality

* prove parachain head at block, selected by headers relay

* const ANCIENT_BLOCK_THRESHOLD

* TODO -> proof

* clippy and spelling

* BatchCallBuilder::build_batch_call() returns Result

* read first proof from two streams

* FailedToFindFinalityProof -> FinalityProofNotFound

* changed select_headers_to_prove to version from PR review

bridges/relays/client-substrate/src/client.rs

@@ -57,6 +57,18 @@ const SUB_API_GRANDPA_AUTHORITIES: &str = "GrandpaApi_grandpa_authorities";
 const SUB_API_TXPOOL_VALIDATE_TRANSACTION: &str = "TaggedTransactionQueue_validate_transaction";
 const MAX_SUBSCRIPTION_CAPACITY: usize = 4096;
 
+/// The difference between best block number and number of its ancestor, that is enough
+/// for us to consider that ancestor an "ancient" block with dropped state.
+///
+/// The relay does not assume that it is connected to the archive node, so it always tries
+/// to use the best available chain state. But sometimes it still may use state of some
+/// old block. If the state of that block is already dropped, relay will see errors when
+/// e.g. it tries to prove something.
+///
+/// By default Substrate-based nodes are storing state for last 256 blocks. We'll use
+/// half of this value.
+pub const ANCIENT_BLOCK_THRESHOLD: u32 = 128;
+
 /// Opaque justifications subscription type.
 pub struct Subscription<T>(pub(crate) Mutex<futures::channel::mpsc::Receiver<Option<T>>>);