[contract-client] refactor (#7978)

* Refactor usage of registry contract

* remove unsed file

* Rename methods & fix comments

* refactor contract_client:

* Intoduced separate traits for asyncronous and syncronous contract clients
* Removed registrar from ethcore::client
* s/ContractClient/AsyncContractClient

* make the tests compile and pass

* omit changes in ethcore

* Use only one trait for ContractClient

* Use an associated type in the trait to determine syncronous or asyncronous communication
* Export the types from contract-client crate
* Document that "A" in the hash correspons to a DNS A Record A.K.A Address Record

* contract_client -> registrar

* address review feedback