leases: Move lease sorting outside of lock

Because the leases were sorted inside UnsafeLeases() the lessor mutex
ended up being locked while the whole map was sorted. This pulls the
soring outside of the lock, per feedback on
etcd-io#9699

lease/lessor.go

@@ -329,14 +329,14 @@ func (le *lessor) unsafeLeases() []*Lease {
 	for _, l := range le.leaseMap {
 		leases = append(leases, l)
 	}
-	sort.Sort(leasesByExpiry(leases))
 	return leases
 }
 
 func (le *lessor) Leases() []*Lease {
 	le.mu.RLock()
 	ls := le.unsafeLeases()
 	le.mu.RUnlock()
+	sort.Sort(leasesByExpiry(ls))
 	return ls
 }
 
@@ -360,6 +360,7 @@ func (le *lessor) Promote(extend time.Duration) {
 
 	// adjust expiries in case of overlap
 	leases := le.unsafeLeases()
+	sort.Sort(leasesByExpiry(leases))
 
 	baseWindow := leases[0].Remaining()
 	nextWindow := baseWindow + time.Second