* DistCp to support checksum validation when copy blocks in parallel
* address review comments
* add checksums comparison test for combine mode
(cherry picked from commit c765584eb231f8482f5b90b7e8f61f9f7a931d09)
(cherry picked from commit b3c14d4132ed6aa871bb88c4f84f3e3d90da6f93)
Conflicts:
hadoop-tools/hadoop-distcp/src/test/java/org/apache/hadoop/tools/util/TestDistCpUtils.java
c1a2b29c0f