MINOR: stream: Move the HTTP txn in an union

MINOR: stream: Move the HTTP txn in an union

The HTTP transaction is moved in an union. For now, it is the only possible
transaction that can be allocated. But that will change. Thanks to this
commit and the next one, it will be possible to deal with different kind of
transactions for a stream.

This patch looks quite huge, but it is more or less a renaming of all
accesses to "txn" field by "txn.http".