Relay PoA lock-funds transactions proofs (#177)

* relay exchange transaction(s)

* fixed Ethereum::get_block_by_hash

* added exchange trace

* fixed method name

* update for new web3

* svyatonik/rust-web3 -> tomusdrw/rust-web3

* if let Some() -> .expect()

* extracted loops in separate functions

* use yaml references (TIL)

* get eth header with transactions

* cargo fmt --all

* Update primitives/ethereum-poa/src/lib.rs

Co-authored-by: Hernando Castano <HCastano@users.noreply.github.com>

* Update relays/ethereum/src/ethereum_exchange.rs

Co-authored-by: Hernando Castano <HCastano@users.noreply.github.com>

* Update relays/ethereum/src/rpc_errors.rs

Co-authored-by: Hernando Castano <HCastano@users.noreply.github.com>

* Update relays/ethereum/src/exchange.rs

Co-authored-by: Hernando Castano <HCastano@users.noreply.github.com>

* removed comment

* Update relays/ethereum/src/ethereum_exchange.rs

Co-authored-by: Hernando Castano <HCastano@users.noreply.github.com>

* Update relays/ethereum/src/ethereum_exchange.rs

Co-authored-by: Hernando Castano <HCastano@users.noreply.github.com>

* module-level comments

* updated readme

* use web3 from crates.io

* added missing fields info in error description

* cargo fmt --all

Co-authored-by: Hernando Castano <HCastano@users.noreply.github.com>