Fix unstable-light-client + ChainHeadBackend tx events (#1865) #5465

Job	Run time
Check unused dependencies	1m 22s
Cargo fmt	21s
Cargo clippy (WASM)	1m 47s
Cargo clippy	6m 4s
Cargo check	11m 57s
Check documentation and run doc tests	4m 29s
Cargo check (WASM)	1m 30s
Test (no_std)	1m 54s
Test (Native)	18m 21s
Test chainhead backend	13m 55s
Test (WASM)	10m 38s
Test (Light Client)	6m 41s
	1h 18m 59s

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