Fixes recordfinder (again)

Lots of complicated stuff going on here, all elements are getting wiped out when a record is updated and the control is getting disposed at the same time. We've created a dedicated variable to store the datalocker name as a string, this represents a small memory leak but a necessary one it seems.
Fixes #2798

modules/backend/formwidgets/recordfinder/assets/js/recordfinder.js

@@ -40,7 +40,7 @@
     RecordFinder.prototype.dispose = function() {
         this.$el.off('dblclick', this.proxy(this.onDoubleClick))
         this.$el.off('dispose-control', this.proxy(this.dispose))
-        this.$el.removeData('oc.someDisposableControl')
+        this.$el.removeData('oc.recordfinder')
 
         this.$el = null
 
@@ -62,15 +62,20 @@
 
     RecordFinder.prototype.updateRecord = function(linkEl, recordId) {
         if (!this.options.dataLocker) return
-        var $locker = $(this.options.dataLocker)
 
-        $locker.val(recordId)
+
+        // Selector name must be used because by the time success runs
+        // - this.options will be disposed
+        // - $locker element will be replaced
+        var locker = this.options.dataLocker
+
+        $(locker).val(recordId)
 
         this.$el.loadIndicator({ opaque: true })
         this.$el.request(this.options.refreshHandler, {
             success: function(data) {
                 this.success(data)
-                $locker.trigger('change')
+                $(locker).trigger('change')
             }
         })