Allow cache.<name>.diskLimit = 0 to disable on disk cache

Being able to make the on disk cache disabled for only some caches
is more useful than being able to make the on disk cache unlimited
in size.  Ehcache isn't that efficient at writing elements out to
the disk, and since this is strictly a cache, it should always have
a reasonable bound placed on its contents.

Signed-off-by: Shawn O. Pearce <sop@google.com>

src/main/java/com/google/gerrit/server/config/CacheManagerProvider.java

@@ -159,8 +159,8 @@ public class CacheManagerProvider implements Provider<CacheManager> {
         int buffer = c.getDiskSpoolBufferSizeMB() * MB;
         buffer = config.getInt("cache", name, "diskbuffer", buffer) / MB;
         c.setDiskSpoolBufferSizeMB(Math.max(1, buffer));
-        c.setOverflowToDisk(true);
-        c.setDiskPersistent(true);
+        c.setOverflowToDisk(c.getMaxElementsOnDisk() > 0);
+        c.setDiskPersistent(c.getMaxElementsOnDisk() > 0);
       }
       return c;
     }