genObjectId slightly off spec

[norpan]

http://docs.mongodb.org/manual/reference/object-id/

ObjectId is a 12-byte BSON type, constructed using:

a 4-byte value representing the seconds since the Unix epoch,
a 3-byte machine identifier,
a 2-byte process id, and
a 3-byte counter, starting with a random value.

But currently the counter always starts at 0.

http://xkcd.com/221/