REORG: conn_stream: move conn-stream stuff in dedicated files

Move code dealing with the conn-streams in dedicated files.
diff --git a/src/mux_fcgi.c b/src/mux_fcgi.c
index d5558a6..374d26e 100644
--- a/src/mux_fcgi.c
+++ b/src/mux_fcgi.c
@@ -17,6 +17,7 @@
 #include <haproxy/api.h>
 #include <haproxy/cfgparse.h>
 #include <haproxy/connection.h>
+#include <haproxy/conn_stream.h>
 #include <haproxy/errors.h>
 #include <haproxy/fcgi-app.h>
 #include <haproxy/fcgi.h>