Message transactions mortality (#1191)

* transactions mortality in message and complex relays

* logging + enable in test deployments

* spellcheck

* fmt

bridges/relays/lib-substrate-relay/src/lib.rs

@@ -18,6 +18,8 @@
 
 #![warn(missing_docs)]
 
+use std::time::Duration;
+
 pub mod conversion_rate_update;
 pub mod error;
 pub mod finality_pipeline;
@@ -28,3 +30,12 @@ pub mod messages_lane;
 pub mod messages_source;
 pub mod messages_target;
 pub mod on_demand_headers;
+
+/// Default relay loop stall timeout. If transactions generated by relay are immortal, then
+/// this timeout is used.
+///
+/// There are no any strict requirements on block time in Substrate. But we assume here that all
+/// Substrate-based chains will be designed to produce relatively fast (compared to the slowest
+/// blockchains) blocks. So 1 hour seems to be a good guess for (even congested) chains to mine
+/// transaction, or remove it from the pool.
+pub const STALL_TIMEOUT: Duration = Duration::from_secs(60 * 60);