_addCurrentTimeEntropy is a private function and should only be used in the library.
I do not want to give any of them a fixed estimatedEntropy because I want to be able to "just add it to the pool" when it is very possible that it has no entropy at all (setTimeout, etc.)
We can anyhow discuss a upper limit to have a difference between Performance.now() and Date.now(). I would also like both of them to have an entropy because I think the Entropy should come from the "random" timepoint and not from possible inner mechanics of Performance.now.