MINOR: http: create a dedicated pool for http_txn

This one will not necessarily be allocated for each stream, and we want
to use the fact that it equals null to know it's not present so that we
can always deduce its presence from the stream pointer.

This commit only creates the new pool.
diff --git a/src/stream.c b/src/stream.c
index d28684e..cd0f8bd 100644
--- a/src/stream.c
+++ b/src/stream.c
@@ -680,6 +680,7 @@
 	/* We may want to free the maximum amount of pools if the proxy is stopping */
 	if (fe && unlikely(fe->state == PR_STSTOPPED)) {
 		pool_flush2(pool2_buffer);
+		pool_flush2(pool2_http_txn);
 		pool_flush2(pool2_hdr_idx);
 		pool_flush2(pool2_requri);
 		pool_flush2(pool2_capture);