MINOR: htx/http-ana: Save info about Upgrade option in the Connection header

MINOR: htx/http-ana: Save info about Upgrade option in the Connection header

Add an HTX start-line flag and its counterpart into the HTTP message to
track the presence of the Upgrade option into the Connection header. This
way, without parsing the Connection header again, it will be easy to know if
a client asks for a protocol upgrade and if the server agrees to do so. It
will also be easy to perform some conformance checks when a
101-switching-protocols is received.