MINOR: cfg-quic: define tune.quic.conn-buf-limit

Add a new global configuration option to set the limit of buffers per
QUIC connection. By default, this value is set to 30.
diff --git a/src/haproxy.c b/src/haproxy.c
index a26c8f3..0187bc6 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -203,6 +203,9 @@
 #else
 		.idle_timer = 1000, /* 1 second */
 #endif
+#ifdef USE_QUIC
+		.quic_streams_buf = 30,
+#endif /* USE_QUIC */
 	},
 #ifdef USE_OPENSSL
 #ifdef DEFAULT_MAXSSLCONN