[Possible Patch] Some locations cause JOSM jnlp to freeze when opening the `Imagery` menu

[taylor.smock]

This may be Mac OS X only.

The issue appears to be on source:trunk/src/org/openstreetmap/josm/tools/ImageProvider.java#L858​ -- cache remains locked by another thread indefinitely.

Is there a reason why we aren't using java.util.concurrent.ConcurrentHashMap instead of java.util.HashMap? Or an actual lock object? I know I've gotten warnings from some lint tools about using synchronized (map) in the past.

Alternatively, we could just wrap it in a Collections.synchronizedMap.

Please note that I have not tested it yet -- I need to figure out how to run a jnlp from a local source. I may end up having to spin up a server for it.

Steps to reproduce:
1) Start josm.jnlp (josm-latest.jnlp works as well)
2) '​https://api.openstreetmap.org/api/0.6/map?bbox=-47.8898656,-15.8114964,-47.8875375,-15.8092976' is the test area
3) Open Imagery menu

[taylor.smock]

Using a ConcurrentHashMap

[taylor.smock]

Using a synchronizedMap