Refactor HttpClient stream setup

closes #16510 
No. The `ResponseParsingMerge` stage will complete to its downstream (which comes in here) if

a) the `methodByPass` has completed and the last response has been fully received
b) or the server has closed its outgoing (our incoming) side of the connection
c) or the response that has been received had protocol HTTP/1.1 and a `Connection: close` header
d) or the response that has been received has protocol HTTP/1.0 and no `Connection: keep-alive` header

Only in one of these cases will the `terminationBackchannelInput` signal completion and the outgoing side will be closed.