add randomness to uid

Hey assaf,

This change adds more randomness to the uid when recording.

Using `Date` alone doesn't work because: 
1) You might have fast requests or many in parallel
2) You might be using timekeeper or the like to "freeze" date and the requests clobber each other

Many APIs require timestamps / nonce so using a library like timekeeper is required in order to freeze the requests. With this change, you're able to freeze time but still record every request.