[CLEANUP] renamed several HTTP structures

Some parts of HTTP processing were incorrectly called "request" while
they are messages or transactions. The following structure members
have changed :

  http_msg.hdr_state => msg_state
  http_msg.sor => som
  http_req.req_state => removed
  http_req => http_txn
diff --git a/src/log.c b/src/log.c
index 52e2ecf..5448c7e 100644
--- a/src/log.c
+++ b/src/log.c
@@ -294,7 +294,7 @@
 	struct proxy *fe = s->fe;
 	struct proxy *be = s->be;
 	struct proxy *prx_log;
-	struct http_req *hreq = &s->hreq;
+	struct http_txn *hreq = &s->txn;
 	int log, tolog;
 	char *uri;
 	char *pxid;