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/proxy.c b/src/proxy.c
index d044150..cd7ba51 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -2145,13 +2145,8 @@
 	/* If the target backend requires HTTP processing, we have to allocate
 	 * the HTTP transaction if we did not have one.
 	 */
-	if (unlikely(!s->txn && be->http_needed)) {
-		if (unlikely(!http_alloc_txn(s)))
-			return 0; /* not enough memory */
-
-		/* and now initialize the HTTP transaction state */
-		http_init_txn(s);
-	}
+	if (unlikely(!s->txn && be->http_needed && !http_create_txn(s)))
+			return 0;
 
 	if (s->txn) {
 		/* If we chain a TCP frontend to an HTX backend, we must upgrade