MINOR: stream-int/txn: Move buffer for L7 retries in the HTTP transaction

MINOR: stream-int/txn: Move buffer for L7 retries in the HTTP transaction

The L7 retries only concerns the stream when a server connection is
established. Thus instead of storing the L7 buffer into the
stream-interface, it may be moved to the stream. And because it is only
available for HTTP streams, it may be moved in the HTTP transaction.

Associated flags are also moved into the HTTP transaction.