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.

See also #8789.

R=@trevnorris?