function uid (len) {
  return byteArrayToBase64(randomBytes(Math.ceil(Math.max(8, len * 2)))).replace(/[+\/]/g, '').slice(0, len);
}
Return a random alphanumerical string of length len
There is a very small probability (less than 1/1,000,000) for the length to be less than len
(il the base64 conversion yields too many pluses and slashes) but
that's not an issue here
The probability of a collision is extremely small (need 3*10^12 documents to have one chance in a million of a collision)
See http://en.wikipedia.org/wiki/Birthday_problem