* Replace MaterializerSettings with Attributes #25559
* Field access to settings deprecated to make stages use attributes instead
* Internal stages updated to use attributes
* Docs on ActorMaterializerSettings updated to recommend away from using it
* Verify all stages stopped after each testcase in FlowGroupBySpec
* Subscription timeout attributes merged into one
Introduces a materializer started through an extension and then an implicit
conversion for Scala turning an implicitly available ActorSystem into a
materializer. The Java APIs has been ammended with run-methods accepting
an ActorSystem.
* Adds internal access to materializer before initialization (#25324)
* Implements new actorRef source based on graph stage (#25324)
* Removes obsolete actorRef source (#25324)
* Improves backwards compatibility with old implementation (#25324)
* Removes dedicated new subclass for materializer access again (#25324)
* Improves implementation (#25324)
* Finalizes implementation (#25324)
* Small improvements to API and documentation (#25324)
* Completion strategy as a replacement for poison pill (#25324)
* Adding more tests and updating the documentation (#25324)
* Add CopyrightHeader support for sbt-boilerplate plugin.
* Add CopyrightHeader support for `*.proto` files.
* Add regex match for both `–` and `-` for CopyrightHeader.
* Add CopyrightHeader support for sbt build files.
* Update copyright from 2018 to 2019.
progressed with cleanup, removing the same thread exec context is
weird... causes issues :-/ Need to debug more, could be that some race
also exists in mapAsync then :\
WIP
finish ask impl via watch stage
mima
consistency spec
fix paradox, and fix adding ask/watch to javadsl source
follow up review
