[SPARK-15541] Casting ConcurrentHashMap to ConcurrentMap (master branch)

## What changes were proposed in this pull request?

Casting ConcurrentHashMap to ConcurrentMap allows to run code compiled with Java 8 on Java 7

## How was this patch tested?

Compilation. Existing automatic tests

Author: Maciej Brynski <maciej.brynski@adpilot.pl>

Closes #14459 from maver1ck/spark-15541-master.

core/src/main/scala/org/apache/spark/rpc/netty/Dispatcher.scala

@@ -17,7 +17,7 @@
 
 package org.apache.spark.rpc.netty
 
-import java.util.concurrent.{ConcurrentHashMap, LinkedBlockingQueue, ThreadPoolExecutor, TimeUnit}
+import java.util.concurrent.{ConcurrentHashMap, ConcurrentMap, LinkedBlockingQueue, ThreadPoolExecutor, TimeUnit}
 import javax.annotation.concurrent.GuardedBy
 
 import scala.collection.JavaConverters._
@@ -42,8 +42,10 @@ private[netty] class Dispatcher(nettyEnv: NettyRpcEnv) extends Logging {
     val inbox = new Inbox(ref, endpoint)
   }
 
-  private val endpoints = new ConcurrentHashMap[String, EndpointData]
-  private val endpointRefs = new ConcurrentHashMap[RpcEndpoint, RpcEndpointRef]
+  private val endpoints: ConcurrentMap[String, EndpointData] =
+    new ConcurrentHashMap[String, EndpointData]
+  private val endpointRefs: ConcurrentMap[RpcEndpoint, RpcEndpointRef] =
+    new ConcurrentHashMap[RpcEndpoint, RpcEndpointRef]
 
   // Track the receivers whose inboxes may contain messages.
   private val receivers = new LinkedBlockingQueue[EndpointData]