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/Add-ability-to-configure-frosted_ice-properties.patch

@@ -9,8 +9,8 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 --- a/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java
 +++ b/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java
 @@ -0,0 +0,0 @@ public class PaperWorldConfig {
-         maxMobSpawnAttempts = getInt("max-mob-spawn-attempts", 50);
-         log( "Max Mob Spawn Attempts: " + maxMobSpawnAttempts);
+             }
+         }
      }
 +
 +    public boolean frostedIceEnabled = true;