Fix sub2eth synchronization (#172)

* ease serde version requirements (to build OE with builtin)

* trace + fix completion notifications

* check incompletion on submit

* fix compilation

* do not ask for synced blocks when queue is empty

* cargo fmt --all

* Update relays/ethereum/src/ethereum_client.rs

Co-authored-by: Tomasz Drwięga <[email protected]>

* remove closure

* fn submit_substrate_header() -> Option<RpcError>

Co-authored-by: Tomasz Drwięga <[email protected]>