watchdog: fix timeout for early polling return

Switch from running the loop with UV_RUN_ONCE to UV_RUN_DEFAULT, because
it's possible that the poll returns earlier than expected and thus the
timer is not run on a single interation.

The loop is not stopped either from the timer callback or from the async
handle's.

Reviewed-By: Julien Gilli <julien.gilli@joyent.com>
PR-URL: https://github.com/joyent/node/pull/9410