FallbackProvider unref timers too aggressively

[ricmoo]

The FallbackProivder unrefs timers (in node.js) to prevent slower responses from preventing exit. This can cause pre-mature exit.

The timers need to be more carefully managed, by unref-ing any timer still in fight once a result is available, but no sooner.

[ricmoo]

This was included in 5.0.0-beta.185.