SourceLayout: rename ClientSocketContext to Http::Stream

SourceLayout: rename ClientSocketContext to Http::Stream

 ... and provided through http/libsquid-http.la.

The name is chosen to match the RFC7540 HTTP/2 "stream" terminology.
Which defines a stream as a bi-directional transaction, including request,
reply and all related 1xx informational and/or control messages.

That same word "stream" is also used in RFC7230 briefly to describe the
same "transaction" scope and details. But not formalized until RFC7540.

Http::Stream's may be initiated by a client HTTP request, Squid internally,
or in HTTP/2 a server PUSH_PROMISE frame.

There are no logic changes in this. Just symbol renaming and move.