!str #15742 Hook-up groupBy to new DSL

* Produces Flows instead of Publisher
!str #15742 Hook-up prefixAndTail to new DSL

* Produces tail with flows instead of Publisher
!str #15742 Hook-up splitWhen to new DSL

* Produces flows instead of publishers
!str #15742 Hook-up flatten (concat all) to new DSL

* input streams of flows, instead of publisher
!hco #15800 mark `Content-Length` header private for now

Instances will only created during HTTP message parsing and its value will
then be provided as a field of one of the HttpEntity subclasses.
=hco fail pending tests faster
=hco #15682 replace default toLowerCase calls with one using Locale.ROOT
Merge pull request #15826 from spray/w/fix-several-small-issues

Fixes for several small issues #15682, #15800
=str #15740 hookup filter and collect

* copy tests from old DSL
* adapt ChainSetup to the new DSL
=str #15735 Prototype of FlowGraph

Add documentation
Merge pull request #15805 from akka/wip-15735-flow-graph-research-patriknw

Prototype of FlowGraph
Merge pull request #15834 from 2m/wip-hook-filter-collect

=str #15740 hookup filter and collect
Merge pull request #15824 from akka/wip-15742-flow-of-flows-patriknw

!str #15742 Hook-up flow of flows to new DSL
=str #15739 Hook-up grouped, groupedWithin, timerTransform to new DSL
=str #15738 Hook-up drop, dropWithin, take, takeWithin to new DSL
=str #15741 Hook-up conflate, expand, buffer, and FoldSink
Merge pull request #15838 from akka/wip-15739-15738-15741-patriknw

=str #15739 #15738 #15741 Hook-up more ops to new DSL
=str #15402 Split up MultiStream(Input|Output)Processor since SSL will use both
+str #15402 SSL support for streams