Use transferTo when copy merge files in ExternalSorter

Since this is a file to file copy, using transferTo should be faster.

Author: Raymond Liu <raymond.liu@intel.com>

Closes #1884 from colorant/externalSorter and squashes the following commits:

6e42f3c [Raymond Liu] More code into copyStream
bfb496b [Raymond Liu] Use transferTo when copy merge files in ExternalSorter

(cherry picked from commit 246cb3f158686348a698d1c0da3001c314727129)
Signed-off-by: Reynold Xin <rxin@apache.org>