[REST] Do not disable block caching when scanning

The REST gateway pessimistically disables block caching when issuing Scans to the cluster, using Scan#setCacheBlocks(false) in ScannerResultGenerator. It does not do this when issuing Gets on behalf of HTTP clients in RowResultGenerator. This is an old idea now, the reasons for doing so lost sometime back in the era when HBase walked the earth with dinosaurs (< 0.20). We probably should not be penalizing REST scans in this way.