=htc fix broken per-connection header cache
=htc replace `merge` + `zip` in server pipeline with new `FlexiMerge`
=htc small simplification in message parsers
+htc minor additions to header model
=htc #15799 add server-side 100-continue support, fix request- and entity-stream error propagation
=htc prevent response entity rendering for status codes not allowing response entities
Merge pull request #16337 from spray/wip-15799-mathias

Add server-side support for `Expect: 100-continue`, other improvements