Elasticsearch version: 5.6+master

Description of the problem including expected versus actual behavior:
_cat/aliases performs very slowly when cluster security is enabled. The issue was reported from a user upgrading 1.7.5 to 5.6.15 who noticed the API call was performing much slower on the later version. @imotov is able to reproduce on master.

Steps to reproduce:

1. enable security
2. create a 90000 aliases
3. call _cat/aliases