Add Gitlab::LDAP::Access.open

This new method wraps Gitlab::LDAP::Adapter.open to enable connection
reuse.

lib/gitlab/ldap/access.rb

 module Gitlab
   module LDAP
     class Access
+      attr_reader :adapter
+
+      def self.open(&block)
+        Gitlab::LDAP::Adapter.open do |adapter|
+          block.call(self.new(adapter))
+        end
+      end
+
+      def initialize(adapter=nil)
+        @adapter = adapter
+      end
+
       def allowed?(user)
-        !!Gitlab::LDAP::Person.find_by_dn(user.extern_uid)
+        !!Gitlab::LDAP::Person.find_by_dn(user.extern_uid, adapter)
       rescue
         false
       end