Finish client refactor

[carpawell]

Finish #323 work. I thought it was done but we still have non-object operations with client generalized context. In other words, contextCall should be removed. #323 proved that it does not save us from additional code, it even requires more code in fact. At the same time, it makes the code more complex: more additional structs, more anonymous funcs, callbacks, internal interfaces, etc.

[cthulhu-rider]

• Cover client package with unit tests #270 first!