Replace counterfeiter package with gomock for mocking

[oleksmir]

The counterfeiter library we use for mocks has a well-known issue about cyclic imports for public interfaces. That leads you to use private interfaces which is not very good for component decoupling.

Despite the fact that counterfeiter is a good tool for mocking, the described bug above makes this tool unusable.

I think using [gomock] is a better choice. It is an official package provided by golang community which does not have problems with mock creation for public interfaces.