RegionScanner.nextRaw(...) should not update metrics

I added the RegionScanner.nextRaw(...) to allow "smart" client to avoid some of the default work that HBase is doing, such as

RegionOperation and synchronized(scanner) for each row.

Metrics should follow the same approach. Collecting them per row is expensive and a caller should have the option to collect those later or to avoid collecting them completely.

We can also save some cycles in RSRcpServices.scan(...) if we updated the metric only once/batch instead of each row.