timers: fix unref() memory leak

The destructor isn't being called for timers that have been unref'd.

Fixes: https://github.com/joyent/node/issues/8364
Signed-off-by: Trevor Norris <trev.norris@gmail.com>
src: reduce cpu profiler overhead

Reduce the overhead of the CPU profiler by suppressing SIGPROF signals
when sleeping / polling for events.  Avoids unnecessary wakeups when
the CPU profiler is active.  Depends on libuv/libuv#15.

Refs strongloop/strong-agent#3 and strongloop-internal/scrum-cs#37.