Allow customizing health-check delay dynamically

[trustin]

From @adriancole at #1910:

forgive the drive by but I guess the state of throwing exception is more a
"fail fast" thing which could be configured and if not then we can consider
how to handle temporary absence of all endpoints.

I dont recall the implementation here but iirc in finagle when fail fast is
off.. the down endpoints, maybe all, would be individual countdown buckets
..

https://github.com/twitter/finagle/blob/76f791937fb0e4ec8ec8cdea48ae001f0591071a/finagle-core/src/main/scala/com/twitter/finagle/liveness/FailureAccrualPolicy.scala