generate within correct random number range

We get 2 bytes for the process id, which is a number within [0, 2^16).
Math.random() returns a number [0, 1), so we don't need to worry about
the value being >= 2^16.
start increment at a random integer

The 3-byte counter should start at a random integer, instead of 0 as per
the ObjectId
[documentation](http://docs.mongodb.org/manual/reference/object-id/).