timers: improve Timer.now() performance
Record the start time so we can make the return value of Timer.now() relative to it, increasing the chances that it fits in a tagged integer instead of a heap-allocated double, at least for the first one or two billion milliseconds.
R=@trevnorris?
CI: https://jenkins-iojs.nodesource.com/view/iojs/job/iojs+any-pr+multi/203/