[streaming] Source sync

[mbalassi]

This builds on #521 and fixes some issues in it:

• The pull request was rebased
• fromCollection and generateSequence and the parallel versions have the expected behaviour
• Removed old Serializable unnecessary mentions from both the batch and streaming execution environments.

I consider this an 0.9 blocker.

[gyfora]

Looks good , but we probably should modify (or add) some tests to test the newly added functions because I think none of the new functionality is tested properly.

[mbalassi]

Thanks for the review. The test coverage is the following:

• The newly introduced StreamExecutionEnvionmentTest is for testing the parallelism issues with the new API.
• The parallel versions in functionality are covered by the ComplexIntegrationTest
• The underlying sources are actually the same for the parallel and non-parallel cases they are just initiated with parallelism one, so I would bother with adding specific test cases for that.