## What changes were proposed in this pull request?

#12339 didn't fix the race condition. MemorySinkSuite is still flaky: https://amplab.cs.berkeley.edu/jenkins/job/spark-master-test-maven-hadoop-2.2/814/testReport/junit/org.apache.spark.sql.streaming/MemorySinkSuite/registering_as_a_table/

Here is an execution order to reproduce it.

| Time        |Thread 1           | MicroBatchThread  |
|:-------------:|:-------------:|:-----:|
| 1 | |  `MemorySink.getOffset` |
| 2 | |  availableOffsets ++= newData (availableOffsets is not changed here)  |
| 3 | addData(newData)      |   |
| 4 | Set `noNewData` to `false` in  processAllAvailable |  |
| 5 | | `dataAvailable` returns `false`   |
| 6 | | noNewData = true |
| 7 | `noNewData` is true so just return | |
| 8 |  assert results and fail | |
| 9 |   | `dataAvailable` returns true so process the new batch |

This PR expands the scope of `awaitBatchLock.synchronized` to eliminate the above race.

## How was this patch tested?

test("stress test"). It always failed before this patch. And it will pass after applying this patch. Ignore this test in the PR as it takes several minutes to finish.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #12582 from zsxwing/SPARK-14579-2.