MEDIUM: streams: Add the ability to retry a request on L7 failure.

When running in HTX mode, if we sent the request, but failed to get the
answer, either because the server just closed its socket, we hit a server
timeout, or we get a 404, 408, 425, 500, 501, 502, 503 or 504 error,
attempt to retry the request, exactly as if we just failed to connect to
the server.

To do so, add a new backend keyword, "retry-on".

It accepts a list of keywords, which can be "none" (never retry),
"conn-failure" (we failed to connect, or to do the SSL handshake),
"empty-response" (the server closed the connection without answering),
"response-timeout" (we timed out while waiting for the server response),
or "404", "408", "425", "500", "501", "502", "503" and "504".

The default is "conn-failure".
diff --git a/include/types/stream_interface.h b/include/types/stream_interface.h
index 61937e0..6b30de5 100644
--- a/include/types/stream_interface.h
+++ b/include/types/stream_interface.h
@@ -83,6 +83,7 @@
 	SI_FL_RXBLK_CONN = 0x00100000,  /* other side is not connected */
 	SI_FL_RXBLK_ANY  = 0x001F0000,  /* any of the RXBLK flags above */
 	SI_FL_RX_WAIT_EP = 0x00200000,  /* stream-int waits for more data from the end point */
+	SI_FL_L7_RETRY   = 0x01000000,  /* The stream interface may attempt L7 retries */
 };
 
 /* A stream interface has 3 parts :
@@ -111,6 +112,7 @@
 	int conn_retries;	/* number of connect retries left */
 	unsigned int hcto;      /* half-closed timeout (0 = unset) */
 	struct wait_event wait_event; /* We're in a wait list */
+	struct buffer l7_buffer; /* To store the data, in case we have to retry */
 };
 
 /* operations available on a stream-interface */