Comply with OSM Tile Usage Policy

[simon04]

JOSM violates the OSM Tile Usage Policy ​https://operations.osmfoundation.org/policies/tiles/

DO NOT send no-cache headers. (Cache-Control: no-cache, Pragma: no-cache etc.)

JOSM sends both headers

GET /11/1085/719.png HTTP/1.1
User-Agent: JOSM/1.5 (15736 SVN en_GB) Linux Arch Linux Java/11.0.6
Cache-Control: no-cache
Accept: text/html, image/png, image/jpeg, image/gif, */*
Accept-Encoding: gzip
If-None-Match: "2ab5b55b0b85f4445b940246b7cac567"
Pragma: no-cache
Host: b.tile.openstreetmap.org
Connection: keep-alive
If-Modified-Since: Mon, 20 Jan 2020 19:51:08 GMT

The behaviour has been introduced in r9228. According to that commit message,

This avoids getting objects from Java Web Start Cache…

We have a few options:

1. Drop the no-cache headers altogether
2. Keep no-cache only for Java Web Start (detect via org.openstreetmap.josm.actions.ShowStatusReportAction#isRunningJavaWebStart)
3. Exclude tile.openstreetmap.org

[simon04]

In 15740/josm:

fix #18588, see #12235 - Comply with OSM Tile Usage Policy

Do not send no-cache headers unless hitting JDK-8146450