[SPARK-8430] ExternalShuffleBlockResolver of shuffle service should support UnsafeShuffleManager

andrewor14 can you take a look?thanks

Author: Lianhui Wang <lianhuiwang09@gmail.com>

Closes #6873 from lianhuiwang/SPARK-8430 and squashes the following commits:

51c47ca [Lianhui Wang] update andrewor's comments
2b27b19 [Lianhui Wang] support UnsafeShuffleManager

network/shuffle/src/main/java/org/apache/spark/network/shuffle/ExternalShuffleBlockResolver.java

@@ -108,7 +108,8 @@ public class ExternalShuffleBlockResolver {
 
     if ("org.apache.spark.shuffle.hash.HashShuffleManager".equals(executor.shuffleManager)) {
       return getHashBasedShuffleBlockData(executor, blockId);
-    } else if ("org.apache.spark.shuffle.sort.SortShuffleManager".equals(executor.shuffleManager)) {
+    } else if ("org.apache.spark.shuffle.sort.SortShuffleManager".equals(executor.shuffleManager)
+      || "org.apache.spark.shuffle.unsafe.UnsafeShuffleManager".equals(executor.shuffleManager)) {
       return getSortBasedShuffleBlockData(executor, shuffleId, mapId, reduceId);
     } else {
       throw new UnsupportedOperationException(