Add statistics to client Scanners

Would be nice to be able to get back some stats from a Scanner object. In particular:

• number of localhost round trips, remote host round trips
• bytes transferred local, bytes transferred remote
• propagate counters from the server side (we have some CFileIterator::IOStatistics stuff which we don't really bubble up anywhere)

This would be especially nice to expose into MR and Impala as counters to see if you're going over the network vs getting the expected locality.