Add lease expiration to garbage collection

Allow setting an expiration label to have the garbage
collector remove an item after the specified time.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

leases/lease.go

@@ -0,0 +1,17 @@
+package leases
+
+import "time"
+
+type LeaseOpt func(*Lease)
+
+type LeaseManager interface {
+	Create(...LeaseOpt) (Lease, error)
+	Delete(Lease) error
+	List(...string) ([]Lease, error)
+}
+
+type Lease struct {
+	ID        string
+	CreatedAt time.Time
+	Labels    map[string]string
+}