[SPARK-14630][Build][Core][SQL][Streaming] Code style: public abstract methods should have explicit return types

[lw-lin]

What changes were proposed in this pull request?

Currently many public abstract methods (in abstract classes as well as traits) don't declare return types explicitly, such as in o.a.s.streaming.dstream.InputDStream:

defstart() // should be: def start(): Unitdefstop() // should be: def stop(): Unit

These methods exist in core, sql, streaming; this PR fixes them.

How was this patch tested?

N/A

Which piece of scala style rule led to the changes?

the rule was added separately in #12396

[srowen]

LGTM. It's good practice to make method return types explicit, especially if they're to be implemented in other classes. This should not change any behavior since it just makes the implicit "Unit" return type explicit.

[SparkQA]

Test build #55809 has finished for PR 12389 at commit d3d83b0.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[rxin]

Merging in master. Thanks.

[lw-lin]

@srowen@rxin thank you for the review & merging! :-)