=pro optimize pull request validation for release-2.3-dev branch

 * enable parallel task execution for everything but tests
 * aggregate only streamAndHttp projects
 * disable publishing
 * don't run extra-build-steps

To be reverted before merging into release-2.3
=str centralize number of runs for randomized stream tests
=str don't run streams test twice because of junit.RunWith annotation
Merge pull request #16185 from spray/w/faster-PR-validation

optimize pull request validation for release-2.3-dev branch