BUG/MEDIUM: proxy: Make sure to destroy the stream on upgrade from TCP to H2

In stream_set_backend(), if we have a TCP stream, and we want to upgrade it
to H2 instead of attempting ot reuse the stream, just destroy the
conn_stream, make sure we don't log anything about the stream, and pretend
we failed setting the backend, so that the stream will get destroyed.
New streams will then be created by the mux, as if the connection just
happened.
This fixes a crash when upgrading from TCP to H2, as the H2 mux totally
ignored the conn_stream provided by the upgrade, as reported in github
issue #196.

This should be backported to 2.0.
diff --git a/include/types/stream.h b/include/types/stream.h
index eb31f06..90819e6 100644
--- a/include/types/stream.h
+++ b/include/types/stream.h
@@ -56,7 +56,7 @@
 #define SF_CURR_SESS	0x00000040	/* a connection is currently being counted on the server */
 /* unused: 0x00000080 */
 #define SF_REDISP	0x00000100	/* set if this stream was redispatched from one server to another */
-/* unused: 0x00000200 */
+#define SF_IGNORE	0x00000200      /* The stream lead to a mux upgrade, and should be ignored */
 #define SF_REDIRECTABLE	0x00000400	/* set if this stream is redirectable (GET or HEAD) */
 #define SF_HTX          0x00000800      /* set if this stream is an htx stream */