use the same timestamp for loading resources within one leafletImage call

I've got a map where some icons appear 100 times. The change avoids loading the same resource again and again, i.e. icon.png?cache=1, icon.png?cache=2, icon.png?cache=3 and so on...
As a result this change speeds up processing. Moreover it avoids caching issues because each time leafletImage() is called, a new timestamp is used.