The polling behavior is designed such that invalidated items should not be polled (IsWorthPolling should return false).
Stake contenders are unique in that a particular contenderId can be invalidated but the underlying contender does not fall
out of the contender set unless its proof becomes invalid, so we cannot simply remove the contenderId as a valid contender.

This patch adds an invalidation status bit that latches on when set so that invalidated contenders will no longer be polled.