This one may be present and may be not, thus it's optional in the image.
The C-binding we use report the field absense in the parsed stream via
the has_xxx field, but in the google docs it's stated, that

	"When a message is parsed, if it does not contain an optional
	 element, the corresponding field in the parsed object is set
	 to the default value for that field."

Thus, I also declare the default value for it to be not zero as 0 is
a valid fanout configuration.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>