[MEDIUM] http: add a new transaction flags indicating if we know the transfer length

[MEDIUM] http: add a new transaction flags indicating if we know the transfer length

It's not enough to know if the connection will be in CLOSE or TUNNEL mode,
we still need to know whether we want to read a full message to a known
length or read it till the end just as in TUNNEL mode. Some updates to the
RFC clarify slightly better the corner cases, in particular for the case
where a non-chunked encoding is used last.

Now we also take care of adding a proper "connection: close" to messages
whose size could not be determined.