MEDIUM: http: move header captures from http_txn to struct stream

The header captures are now general purpose captures since tcp rules
can use them to capture various contents. That removes a dependency
on http_txn that appeared in some sample fetch functions and in the
order by which captures and http_txn were allocated.

Interestingly the reset of the header captures were done at too many
places as http_init_txn() used to do it while it was done previously
in every call place.
diff --git a/src/hlua.c b/src/hlua.c
index 650051a..e54c029 100644
--- a/src/hlua.c
+++ b/src/hlua.c
@@ -2165,14 +2165,14 @@
 
 	/* XXX: ? */
 	socket->s->pend_pos = NULL;
+	socket->s->req_cap = NULL;
+	socket->s->res_cap = NULL;
 
 	/* XXX: See later. */
 	socket->s->txn.sessid = NULL;
 	socket->s->txn.srv_cookie = NULL;
 	socket->s->txn.cli_cookie = NULL;
 	socket->s->txn.uri = NULL;
-	socket->s->txn.req.cap = NULL;
-	socket->s->txn.rsp.cap = NULL;
 	socket->s->txn.hdr_idx.v = NULL;
 	socket->s->txn.hdr_idx.size = 0;
 	socket->s->txn.hdr_idx.used = 0;