make sourcegraph support redis password

Created by: alexandnpu

Test plan:

sourcegraph should be able to support redis with authorization.

Note: I failed to setup my dev environment to test on my own mac, as the dev setup is not that easy enough for only the server side. Maybe this PR is an idea to show you that sourcegraph should be able to use password to connect redis. Because in my company, every redis server should use password to connect.