Disallow overlapping visits

Set the `visits.allowOverlappingVisits` global property to `false` to prevent overlapping visits. This property is `true` by default in the core.

distro/configuration/globalproperties/globalproperties-core_data.xml

@@ -11,6 +11,10 @@
         <globalProperty>
             <property>visits.assignmentHandler</property>
             <value>org.openmrs.api.handler.ExistingVisitAssignmentHandler</value>
+        </globalProperty>    
+        <globalProperty>
+            <property>visits.allowOverlappingVisits</property>
+            <value>false</value>
         </globalProperty>
     </globalProperties>
-</config>
+</config>