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/proto_http.h b/include/types/proto_http.h
index 158f094..665ec75 100644
--- a/include/types/proto_http.h
+++ b/include/types/proto_http.h
@@ -22,9 +22,9 @@
 #ifndef _TYPES_PROTO_HTTP_H
 #define _TYPES_PROTO_HTTP_H
 
+#include <common/chunk.h>
 #include <common/config.h>
 
-#include <types/buffers.h>
 #include <types/hdr_idx.h>
 
 /* These are the flags that are found in txn->flags */