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.

"outer loop" is misleading now that there really is an outer loop here. It might say "try to resend the rpc" or something.