add failures time limit

Adds a time limit setting within which all `failures` must occur, otherwise `failures` will be reset to its original value.  Avoids a situation where, if `failures` is set to three, and one failure occurs today, one tomorrow, and another in five days, that these three failures don't cause the node to be marked dead or removed.  Instead, the idea is that the three failures should occur in a similar time frame, such as all within ten minutes of each other.