During profiling of a structured streaming application with Kafka as the source, I came across this exception:

![Structured Streaming Kafka Exceptions](https://user-images.githubusercontent.com/3448320/29743366-4149ef78-8a99-11e7-94d6-f0cbb691134a.png)

This is a 1 minute sample, which caused 106K `NonLocalReturnControl` exceptions to be thrown.
This happens because `CachedKafkaConsumer.get` is ran inside:

`private def runUninterruptiblyIfPossible[T](body: => T): T`

Where `body: => T` is the `get` method. Turning the method into a function means that in order to escape the `while` loop defined in `get` the runtime has to do dirty tricks which involve throwing the above exception.

## What changes were proposed in this pull request?

Instead of using `return` (which is generally not recommended in Scala), we place the result of the `fetchData` method inside a local variable and use a boolean flag to indicate the status of fetching data, which we monitor as our predicate to the `while` loop.

## How was this patch tested?

I've ran the `KafkaSourceSuite` to make sure regression passes. Since the exception isn't visible from user code, there is no way (at least that I could think of) to add this as a test to the existing suite.

Author: Yuval Itzchakov <yuval.itzchakov@clicktale.com>

Closes #19059 from YuvalItzchakov/master.