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.

This could extend a ranged request beyond the limit the caller intended. I think the correct code here should be:

    if desc.EndKey.Less(args.Header().EndKey) {
      args.Header().EndKey = desc.EndKey
    }