reverseproxy: Response buffering & configurable buffer size

Proxy response bodies can now be buffered, and the size of the request body and
response body buffer can be limited. Any remaining content that doesn't fit in the
buffer will remain on the wire until it can be read; i.e. bodies are not truncated,
even if the buffer is not big enough.

This fulfills a customer requirement. This was made possible by their sponsorship!

modules/caddyhttp/reverseproxy/caddyfile.go

@@ -499,6 +499,25 @@ func (h *Handler) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 				}
 				h.BufferRequests = true
 
+			case "buffer_responses":
+				if d.NextArg() {
+					return d.ArgErr()
+				}
+				h.BufferResponses = true
+
+			case "max_buffer_size":
+				if !d.NextArg() {
+					return d.ArgErr()
+				}
+				size, err := strconv.Atoi(d.Val())
+				if err != nil {
+					return d.Errf("invalid size (bytes): %s", d.Val())
+				}
+				if d.NextArg() {
+					return d.ArgErr()
+				}
+				h.MaxBufferSize = int64(size)
+
 			case "header_up":
 				var err error