[SPARK-19564][SPARK-19559][SS][KAFKA] KafkaOffsetReader's consumers should not be in the same group

## What changes were proposed in this pull request?

In `KafkaOffsetReader`, when error occurs, we abort the existing consumer and create a new consumer. In our current implementation, the first consumer and the second consumer would be in the same group (which leads to SPARK-19559), **_violating our intention of the two consumers not being in the same group._**

The cause is that, in our current implementation, the first consumer is created before `groupId` and `nextId` are initialized in the constructor. Then even if `groupId` and `nextId` are increased during the creation of that first consumer, `groupId` and `nextId` would still be initialized to default values in the constructor for the second consumer.

We should make sure that `groupId` and `nextId` are initialized before any consumer is created.

## How was this patch tested?

Ran 100 times of `KafkaSourceSuite`; all passed

Author: Liwei Lin <lwlin7@gmail.com>

Closes #16902 from lw-lin/SPARK-19564-.

external/kafka-0-10-sql/src/main/scala/org/apache/spark/sql/kafka010/KafkaOffsetReader.scala

@@ -64,6 +64,13 @@ private[kafka010] class KafkaOffsetReader(
   })
   val execContext = ExecutionContext.fromExecutorService(kafkaReaderThread)
 
+  /**
+   * Place [[groupId]] and [[nextId]] here so that they are initialized before any consumer is
+   * created -- see SPARK-19564.
+   */
+  private var groupId: String = null
+  private var nextId = 0
+
   /**
    * A KafkaConsumer used in the driver to query the latest Kafka offsets. This only queries the
    * offsets and never commits them.
@@ -76,10 +83,6 @@ private[kafka010] class KafkaOffsetReader(
   private val offsetFetchAttemptIntervalMs =
     readerOptions.getOrElse("fetchOffset.retryIntervalMs", "1000").toLong
 
-  private var groupId: String = null
-
-  private var nextId = 0
-
   private def nextGroupId(): String = {
     groupId = driverGroupIdPrefix + "-" + nextId
     nextId += 1