=str #16041 Move PersistentSource to the latest Stream DSL

* implemented as an extension of KeyedActorFlowSource
Merge pull request #16133 from 2m/wip-persistent-source-16041

=str #16041 Move PersistentSource to the latest Stream DSL
+str #16114 better consistency specs and missing methods added - javadsl
Merge pull request #16116 from ktoso/wip-consistency-dsl-ktoso

+str #16114 DSL consistency (factories)
!str #16039 Remove old scaladsl, rename scaladsl2

* and impl2, testkit2
* keeping io2 for now
Merge pull request #16154 from akka/wip-16039-rm-old-dsl-patriknw

!str #16039 Remove old scaladsl, rename scaladsl2
!str #16069 disallows using one Keyed sink/source ambiguously (many times)