Making client class under its service interface namespace

[kanghyojun]

The compiler currently generates a pair of an interface and its client class for each service e.g.:

class FooService(Service):
    ...

class FooService_Client(FooService):
    ...

Making client classes under their interface namespace would be better for avoiding namespace pollution e.g.:

class FooService(Service):
    ...

class FooService_Client(FooService):
    ...

FooService.Client = FooService_Client