[SPARK-4441] Close Tachyon client when TachyonBlockManager is shutdown

Currently Tachyon client is not closed when TachyonBlockManager is shut down. which causes some resources in Tachyon not reclaimed

Author: Mingfei <mingfei.shi@intel.com>

Closes #3299 from shimingfei/closeClient and squashes the following commits:

0913fbd [Mingfei] close Tachyon client when TachyonBlockManager is shutdown

core/src/main/scala/org/apache/spark/storage/TachyonBlockManager.scala

@@ -148,6 +148,7 @@ private[spark] class TachyonBlockManager(
               logError("Exception while deleting tachyon spark dir: " + tachyonDir, e)
           }
         }
+        client.close()
       }
     })
   }