[SPARK-2485][SQL] Lock usage of hive client.

Author: Michael Armbrust <michael@databricks.com>

Closes #1412 from marmbrus/lockHiveClient and squashes the following commits:

4bc9d5a [Michael Armbrust] protected[hive]
22e9177 [Michael Armbrust] Add comments.
7aa8554 [Michael Armbrust] Don't lock on hive's object.
a6edc5f [Michael Armbrust] Lock usage of hive client.

sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveMetastoreCatalog.scala

@@ -43,14 +43,15 @@ import scala.collection.JavaConversions._
 private[hive] class HiveMetastoreCatalog(hive: HiveContext) extends Catalog with Logging {
   import HiveMetastoreTypes._
 
-  val client = Hive.get(hive.hiveconf)
+  /** Connection to hive metastore.  Usages should lock on `this`. */
+  protected[hive] val client = Hive.get(hive.hiveconf)
 
   val caseSensitive: Boolean = false
 
   def lookupRelation(
       db: Option[String],
       tableName: String,
-      alias: Option[String]): LogicalPlan = {
+      alias: Option[String]): LogicalPlan = synchronized {
     val (dbName, tblName) = processDatabaseAndTableName(db, tableName)
     val databaseName = dbName.getOrElse(hive.sessionState.getCurrentDatabase)
     val table = client.getTable(databaseName, tblName)