fix overflow bug in nats_handler.go / nats.CustomReconnectDelay()
fix overflow bug in nats_handler.go / nats.CustomReconnectDelay()

fix for https://github.com/cloudfoundry/bosh-agent/issues/289
- avoid arithmetic overflow with math.Pow and subsequent conversion to int64 (by limiting exponent to a safe value)
- add sanity check for reconnect delay to safeguard against negative values or zero
- add support for delay in non-integer seconds (by fixing how time.Duration() is used with time.Second)
- use more consistent naming of constants and variables

cloudfoundry/bosh-agent,cloudfoundry/bosh-agent