The golang/time package tracks monotonic time in each time.Time
returned by time.Now function for Go 1.9.
Use time.Time to measure whether a lease is expired and remove
previous pkg/monotime. Use zero time.Time to mean forever. No
expiration when expiry.IsZero() is true.
pkg/ is a collection of utility packages used by etcd without being specific to etcd itself. A package belongs here
only if it could possibly be moved out into its own repository in the future.