MAJOR: http: move http_txn out of struct stream

MAJOR: http: move http_txn out of struct stream

Now this one is dynamically allocated. It means that 280 bytes of memory
are saved per TCP stream, but more importantly that it will become
possible to remove the l7 pointer from fetches and converters since
it will be deduced from the stream and will support being null.

A lot of care was taken because it's easy to forget a test somewhere,
and the previous code used to always trust s->txn for being valid, but
all places seem to have been visited.

All HTTP fetch functions check the txn first so we shouldn't have any
issue there even when called from TCP. When branching from a TCP frontend
to an HTTP backend, the txn is properly allocated at the same time as the
hdr_idx.