[SPARK-18729][SS] Move DataFrame.collect out of synchronized block in MemorySink

## What changes were proposed in this pull request?

Move DataFrame.collect out of synchronized block so that we can query content in MemorySink when `DataFrame.collect` is running.

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #16162 from zsxwing/SPARK-18729.

(cherry picked from commit 1b2785c3)
Signed-off-by: Tathagata Das <tathagata.das1565@gmail.com>