+str add akka.stream.materializer.file-io-dispatcher setting
+htp #15922 import FileAndResourceDirectives from spray
!htc,par cleanup warnings

 - about Rendering not being accessible for implementors
 - about DepFn1/2 being defined in package object
=pro disable '-Xlint' in akka-http/-tests for Scala 2.10
=pro fix the parallelExecution setting broken in #16185