[SPARK-19267][SS] Fix a race condition when stopping StateStore

## What changes were proposed in this pull request?

There is a race condition when stopping StateStore which makes `StateStoreSuite.maintenance` flaky. `StateStore.stop` doesn't wait for the running task to finish, and an out-of-date task may fail `doMaintenance` and cancel the new task. Here is a reproducer: https://github.com/zsxwing/spark/commit/dde1b5b106ba034861cf19e16883cfe181faa6f3

This PR adds MaintenanceTask to eliminate the race condition.

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>
Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #16627 from zsxwing/SPARK-19267.