Allow operations to cross range boundaries

 * add proto.Combinable interface

Combinable is implemented by responses whose requests may access multiple ranges,
such as {Scan,DeleteRange}Response and allows multiple responses to be merged
into a single one transparently.

* allow multi-range Scan and DeleteRange

adapted the distributed sender to split such requests up
and adress them to the individual ranges.
The Combinable interface is used to create a single response
from those parts which is returned with the client.Call.

It's actually idiomatic go to call methods with a nil pointer. It can be incredibly useful, as it allows the caller to avoid nil checks. Usually it works something like this:

func (f *Foo) Bar() Baz {
  if f == nil {
    return defaultBaz
  }
  ...
}