circuit breaker for failed nodes

[thraxil]

apply the circuit breaker pattern to handling of failed nodes to prevent thundering herd problems. Basically, if pinging (or writing to a node) fails, we set its last_failed and wait announce_frequency seconds to try pinging again. If it fails a second ping, we wait 2 * announce_frequency seconds before making the next attempt, then 3 * announce_frequency times, and so on.