MINOR: http-ana: Simplify creation/destruction of HTTP transactions

Now allocation and initialization of HTTP transactions are performed in a
unique function. Historically, there were two functions because the same TXN
was reset for K/A connections in the legacy HTTP mode. Now, in HTX, K/A
connections are handled at the mux level. A new stream, and thus a new TXN,
is created for each request. In addition, the function responsible to end
the TXN is now also reponsible to release it.

So, now, http_create_txn() and http_destroy_txn() must be used to create and
destroy an HTTP transaction.
diff --git a/src/frontend.c b/src/frontend.c
index 93c8a86..a8ed2c4 100644
--- a/src/frontend.c
+++ b/src/frontend.c
@@ -145,17 +145,8 @@
 			goto out_free_reqcap;	/* no memory */
 	}
 
-	if (fe->http_needed || IS_HTX_STRM(s)) {
-		/* we have to allocate header indexes only if we know
-		 * that we may make use of them. This of course includes
-		 * (mode == PR_MODE_HTTP).
-		 */
-		if (unlikely(!http_alloc_txn(s)))
-			goto out_free_rspcap; /* no memory */
-
-		/* and now initialize the HTTP transaction state */
-		http_init_txn(s);
-	}
+	if ((fe->http_needed || IS_HTX_STRM(s)) && !http_create_txn(s))
+		goto out_free_rspcap;
 
 	/* everything's OK, let's go on */
 	return 1;