cluster: support request/response within a cluster

/to @kraman 

Destined for strong-supervisor, so it can ask its workers to start and stop profiling.
Symmetrical with client, and server: it allows usage like:


```
// in a client, when you won't be using cluster or server:
require('strong-control-channel/client')
// etc.
```

Its also supported to just use .X on the main module.

Seemed useful to me, but if its just confusing syntax sugar, I can leave only the properties on the main module as the way to get the various parts.