[SPARK-7519] [SQL] fix minor bugs in thrift server UI

Bugs description:

1. There are extra commas on the top of session list.
2. The format of time in "Start at:" part is not the same as others.
3. The total number of online sessions is wrong.

Author: tianyi <tianyi.asiainfo@gmail.com>

Closes apache#6048 from tianyi/SPARK-7519 and squashes the following commits:

ed366b7 [tianyi] fix bug

sql/hive-thriftserver/src/main/scala/org/apache/spark/sql/hive/thriftserver/HiveThriftServer2.scala

@@ -147,7 +147,7 @@ object HiveThriftServer2 extends Logging {
     override def onApplicationEnd(applicationEnd: SparkListenerApplicationEnd): Unit = {
       server.stop()
     }
-
+    var onlineSessionNum: Int = 0
     val sessionList = new mutable.LinkedHashMap[String, SessionInfo]
     val executionList = new mutable.LinkedHashMap[String, ExecutionInfo]
     val retainedStatements =
@@ -170,11 +170,13 @@ object HiveThriftServer2 extends Logging {
     def onSessionCreated(ip: String, sessionId: String, userName: String = "UNKNOWN"): Unit = {
       val info = new SessionInfo(sessionId, System.currentTimeMillis, ip, userName)
       sessionList.put(sessionId, info)
+      onlineSessionNum += 1
       trimSessionIfNecessary()
     }
 
     def onSessionClosed(sessionId: String): Unit = {
       sessionList(sessionId).finishTimestamp = System.currentTimeMillis
+      onlineSessionNum -= 1
     }
 
     def onStatementStart(

sql/hive-thriftserver/src/main/scala/org/apache/spark/sql/hive/thriftserver/ui/ThriftServerPage.scala

@@ -29,7 +29,7 @@ import org.apache.spark.ui.UIUtils._
 import org.apache.spark.ui._
 
 
-/** Page for Spark Web UI that shows statistics of a streaming job */
+/** Page for Spark Web UI that shows statistics of a thrift server */
 private[ui] class ThriftServerPage(parent: ThriftServerTab) extends WebUIPage("") with Logging {
 
   private val listener = parent.listener
@@ -42,20 +42,20 @@ private[ui] class ThriftServerPage(parent: ThriftServerTab) extends WebUIPage(""
       generateBasicStats() ++
       <br/> ++
       <h4>
-        {listener.sessionList.size} session(s) are online,
+        {listener.onlineSessionNum} session(s) are online,
         running {listener.totalRunning} SQL statement(s)
       </h4> ++
       generateSessionStatsTable() ++
       generateSQLStatsTable()
     UIUtils.headerSparkPage("ThriftServer", content, parent, Some(5000))
   }
 
-  /** Generate basic stats of the streaming program */
+  /** Generate basic stats of the thrift server program */
   private def generateBasicStats(): Seq[Node] = {
     val timeSinceStart = System.currentTimeMillis() - startTime.getTime
     <ul class ="unstyled">
       <li>
-        <strong>Started at: </strong> {startTime.toString}
+        <strong>Started at: </strong> {formatDate(startTime)}
       </li>
       <li>
         <strong>Time since start: </strong>{formatDurationVerbose(timeSinceStart)}
@@ -148,7 +148,7 @@ private[ui] class ThriftServerPage(parent: ThriftServerTab) extends WebUIPage(""
         <tr>
           <td> {session.userName} </td>
           <td> {session.ip} </td>
-          <td> <a href={sessionLink}> {session.sessionId} </a> </td>,
+          <td> <a href={sessionLink}> {session.sessionId} </a> </td>
           <td> {formatDate(session.startTimestamp)} </td>
           <td> {if(session.finishTimestamp > 0) formatDate(session.finishTimestamp)} </td>
           <td> {formatDurationOption(Some(session.totalTime))} </td>