Introduce mockable `ChainSync` object for testing (#12480)

* Introduce mockable `ChainSync` object for testing

`mockall` allows to mock `ChainSync` and to verify that the calls made
to `ChaiSync` are firstly executed at all, that they're executed in
correct order and with correct parameters.

This allows to verify, e.g., that delegating calls directly to
`ChainSync` from `NetworkService` still calls the correct functions with
correct arguments even if `Protocol` middleman is removed.

* Add Cargo.lock

* Fix tests

* Update client/network/Cargo.toml

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update Cargo.lock

* Fix clippy and documentation

Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: parity-processbot <>