[SPARK-15202][SPARKR] add dapplyCollect() method for DataFrame in SparkR.

## What changes were proposed in this pull request?

dapplyCollect() applies an R function on each partition of a SparkDataFrame and collects the result back to R as a data.frame.
```
dapplyCollect(df, function(ldf) {...})
```

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

Author: Sun Rui <sunrui2016@gmail.com>

Closes #12989 from sun-rui/SPARK-15202.