Improve random visitor id generation

[julienmoumne]

A few improvements can be made to match the way the JavaScript tracker generates random identifiers.

This ticket addresses the fact the current implementation is dependent on the system local since Encoding.Default is being used.

We will also use SHA1 instead of MD5 to hash the UUID.

Further improvements could be made to better match the JavaScript tracker such as using information collected on the browser agent such as user agent and enabled plugins as in generateRandomUuid