Add better control over expression timeouts. (#451)

Add better control over expression timeouts.
* //timeout command can be used to change player's current timeout.
* Config now also has a max timeout, can be bypassed with permission
* Timeout of < 0 will let expressions run indefinitely.
* Said expressions won't run on a separate thread, slightly reducing the
  overhead from context switching. For large //gen commands, for example,
  this can actually increase speed.

worldedit-core/src/main/java/com/sk89q/worldedit/LocalConfiguration.java

@@ -67,6 +67,7 @@ public abstract class LocalConfiguration {
     public int navigationWandMaxDistance = 50;
     public int scriptTimeout = 3000;
     public int calculationTimeout = 100;
+    public int maxCalculationTimeout = 300;
     public Set<String> allowedDataCycleBlocks = new HashSet<>();
     public String saveDir = "schematics";
     public String scriptsDir = "craftscripts";