bump the default maxMobSpawns default to 250, and add support for unlimited

Use -1 to represent vanilla/unlimited.
Updated PaperWorldConfig to also update the individual worlds limit if it was set
to the new default value.

Should hopefully help #235

Spigot-Server-Patches/Paper-config-files.patch

@@ -66,8 +66,8 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +
 +        commands = new HashMap<String, Command>();
 +
-+        version = getInt("config-version", 9);
-+        set("config-version", 9);
++        version = getInt("config-version", 10);
++        set("config-version", 10);
 +        readConfig(PaperConfig.class, null);
 +    }
 +
@@ -217,6 +217,9 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +
 +    private void set(String path, Object val) {
 +        config.set("world-settings.default." + path, val);
++        if (config.get("world-settings." + worldName + "." + path) != null) {
++            config.set("world-settings." + worldName + "." + path, val);
++        }
 +    }
 +
 +    private boolean getBoolean(String path, boolean def) {