util.dbuffer: dynamic string buffer

Similar to util.ringbuffer (and shares almost identical API). Differences:

- size limit is optional and dynamic
- does not allocate a fixed buffer of max_size bytes
- focus on simply storing references to existing string objects where possible,
  avoiding unnecessary allocations
- references are still stored in a ring buffer to enable use as a fast FIFO

Optional second parameter to new() provides the number of ring buffer segments. On
Lua 5.2 on my laptop, a segment is ~19 bytes. If the ring buffer fills up, the next
write will compact all strings into a single item.

util/queue.lua

@@ -51,6 +51,13 @@ local function new(size, allow_wrapping)
 			end
 			return t[tail];
 		end;
+		replace = function (self, data)
+			if items == 0 then
+				return self:push(data);
+			end
+			t[tail] = data;
+			return true;
+		end;
 		items = function (self)
 			return function (_, pos)
 				if pos >= items then