Lower entropy for RUM IDs and Timestamps

[chicharr]

There are different sources of entropy in RUM data, that bring much more entropy than what is actually needed. Which leads to:

• more data storage required
• potential de-anonymization

In an effort to reduce the entropy we will:

• reduce the RUM ID to a 6 digits random string, encoded in base64
• add a property t = <milliseconds since first read> to the RUM beacon