REORG: buffers: split buffers into chunk,buffer,channel

Many parts of the channel definition still make use of the "buffer" word.
diff --git a/include/types/session.h b/include/types/session.h
index 9180a94..cce55a4 100644
--- a/include/types/session.h
+++ b/include/types/session.h
@@ -31,7 +31,7 @@
 #include <common/config.h>
 #include <common/mini-clist.h>
 
-#include <types/buffers.h>
+#include <types/channel.h>
 #include <types/proto_http.h>
 #include <types/proxy.h>
 #include <types/queue.h>