Fix "The data on your client does not match the data on the server" error on zoning (#272)

common/src/main/scala/net/psforever/objects/zones/ZoneMap.scala

@@ -33,6 +33,7 @@ class ZoneMap(private val name : String) {
   private var linkDoorLock : Map[Int, Int] = Map()
   private var linkObjectBase : Map[Int, Int] = Map()
   private var buildings : Map[(Int, Int), FoundationBuilder] = Map()
+  private var checksum : Long = 0
 
   def Name : String = name
 
@@ -43,6 +44,13 @@ class ZoneMap(private val name : String) {
     Scale
   }
 
+  def Checksum : Long = checksum
+
+  def Checksum_=(value : Long) : Long = {
+    checksum = value
+    Checksum
+  }
+
   /**
     * The list of all server object builder wrappers that have been assigned to this `ZoneMap`.
     * @return the `List` of all `ServerObjectBuilders` known to this `ZoneMap`