Cache LDAP check in Gitlab::UserAccess

This changes the number of LDAP calls when users access GitLab via
Git-over-SSH or the API. LDAP check results are cached for 1 hour.

lib/gitlab/user_access.rb

@@ -3,13 +3,8 @@ module Gitlab
     def self.allowed?(user)
       return false if user.blocked?
-      if Gitlab.config.ldap.enabled
-        if user.ldap_user?
-          # Check if LDAP user exists and match LDAP user_filter
-          Gitlab::LDAP::Access.open do |adapter|
-            return false unless adapter.allowed?(user)
-          end
-        end
+      if user.requires_ldap_check?
+        return false unless Gitlab::LDAP::Access.allowed?(user)
       end
       true