+str #15174 Add dispatcher setting to FlowMaterializer

* Verify dispatcher with custom default mailbox
Merge pull request #15183 from akka/wip-15174-flow-dispatcher-patriknw

+str #15174 Add dispatcher setting to FlowMaterializer
+str #15172 Add javadsl.Duct as free-standing Flow

* foreach Void
Merge pull request #15181 from akka/wip-15172-javadsl-duct-patriknw

+str #15172 Add javadsl.Duct as free-standing Flow
=str #15221 Fix typo in scaladoc
Merge pull request #15221 from guersam/wip-fix-typo

=str #15221 fix typo in scaladoc
+str : Various cleanups of internal streams architecture

 - factor out receive blocks for inputs and outputs
 - pump uses TransferPhases and become
 - Unification of Transformer and RecoveryTransformer
Merge pull request #15194 from drewhk/wip-stream-cleanups-drewhk

+str Various cleanups of internal streams architecture
Fixes #15158
+str #15197 Add flow.append(duct)