Incorrect lock used in Since/Until methods of Simulator

[nnikolash]

Since and Until methods of simulator are using exclusive lock mode, which is not required here:

// Since returns the time elapsed since t.
func (s *Simulator) Since(t time.Time) time.Duration {
	s.Lock()
	defer s.Unlock()
	return s.now.Sub(t)
}

// Until returns the duration until t.
func (s *Simulator) Until(t time.Time) time.Duration {
	s.Lock()
	defer s.Unlock()
	return t.Sub(s.now)
}

It can be replaced with shared lock, as done in Now method:

// Now returns the current time.
func (s *Simulator) Now() time.Time {
	s.RLock()
	defer s.RUnlock()
	return s.now
}