timers: use uv_now instead of Date.now

This saves a few calls to gettimeofday which can be expensive, and
potentially subject to clock drift. Instead use the loop time which
uses hrtime internally.

fixes #5497
dtrace: unify dtrace and systemtap interfaces
systemtap: add tapset for node user probes

This makes systemtap tracing prettier, you can do things like `stap -e
'probe node_http_server_request { println(probestr); }'`