[SPARK-11244][SPARKR] sparkR.stop() should remove SQLContext

SparkR should remove `.sparkRSQLsc` and `.sparkRHivesc` when `sparkR.stop()` is called. Otherwise even when SparkContext is reinitialized, `sparkRSQL.init` returns the stale copy of the object and complains:

```r
sc <- sparkR.init("local")
sqlContext <- sparkRSQL.init(sc)
sparkR.stop()
sc <- sparkR.init("local")
sqlContext <- sparkRSQL.init(sc)
sqlContext
```
producing
```r
Error in callJMethod(x, "getClass") :
  Invalid jobj 1. If SparkR was restarted, Spark operations need to be re-executed.
```

I have added the check and removal only when SparkContext itself is initialized. I have also added corresponding test for this fix. Let me know if you want me to move the test to SQL test suite instead.

p.s. I tried lint-r but ended up a lots of errors on existing code.

Author: Forest Fang <forest.fang@outlook.com>

Closes #9205 from saurfang/sparkR.stop.