[SPARK-15541] Casting ConcurrentHashMap to ConcurrentMap (branch-1.6)

## 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 #14390 from maver1ck/spark-15541.

Author: Maciej Brynski

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/Catalog.scala

@@ -17,7 +17,7 @@
 
 package org.apache.spark.sql.catalyst.analysis
 
-import java.util.concurrent.ConcurrentHashMap
+import java.util.concurrent.{ConcurrentHashMap, ConcurrentMap}
 
 import scala.collection.JavaConverters._
 import scala.collection.mutable
@@ -80,7 +80,8 @@ trait Catalog {
 }
 
 class SimpleCatalog(val conf: CatalystConf) extends Catalog {
-  private[this] val tables = new ConcurrentHashMap[String, LogicalPlan]
+  private[this] val tables: ConcurrentMap[String, LogicalPlan] =
+    new ConcurrentHashMap[String, LogicalPlan]
 
   override def registerTable(tableIdent: TableIdentifier, plan: LogicalPlan): Unit = {
     tables.put(getTableName(tableIdent), plan)