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.

Move this check to the top and return a non-fatal error. The code here would exit the process on bad user input; you can easily imagine someone adding a bogus range to a non-ranged operation causing us to make one call per range spanned (for something like a get).