SERVER-12539 Make mongorestore not interfere with ongoing user or role modifications

This is done by making mongorestore insert the users/roles from the dump files into temporary
collections that are later merged into the real admin.system.users and admin.system.roles
collections using a new command: _mergeAuthzCollections.
SERVER-12854 Prevent mongorestore from restoring user data to a 2.6 system with 2.4 schema users
fix single-db restores without the --restoreDbUsersAndRoles flag