## What changes were proposed in this pull request?

Make sure accessing mutable variables in MemoryStream and MemorySink are protected by `synchronized`.
This is probably why MemorySinkSuite failed here: https://amplab.cs.berkeley.edu/jenkins/job/spark-master-test-sbt-hadoop-2.2/650/testReport/junit/org.apache.spark.sql.streaming/MemorySinkSuite/registering_as_a_table/

## How was this patch tested?
Existing unit tests.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #12261 from zsxwing/memory-race-condition.