Extract (headers, exchange, messages) relay loops into separate crates (#357)

* extracted relay crates

* moved metrics to utils

* exchange-relay compilation

* fix compilation of headers-relay

* fixed messages-relay compilation

* fixed ethereum-poa-relay compilation

* cargo lock

* cargo fmt --all

* clippy

* cargo fmt --all

* fix tests compilation

* clippy

* eof

* module level docs

* removed obsolete comment

* #![warn(missing_docs)]

* .0 -> Deref

* post-merge fix

* cargo fmt

* Update relays/headers-relay/src/headers.rs

Co-authored-by: Hernando Castano <[email protected]>

* Update relays/headers-relay/src/headers.rs

Co-authored-by: Hernando Castano <[email protected]>

* Update relays/headers-relay/src/lib.rs

Co-authored-by: Hernando Castano <[email protected]>

Co-authored-by: Hernando Castano <[email protected]>