[keyserver] Defer Redis publisher construction until actually used

Summary:
The current behavior means that anything that inadvertedly imports redis.js (update-geoip.js, for instance) will crash if it can't reach Redis.

This diff makes it so RedisPublisher only tries to connect to Redis on the first call to sendMessage().

Test Plan: Make sure yarn update-geoip works from a Docker buildscript

Reviewers: palys-swm, atul

Reviewed By: palys-swm

Subscribers: Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3870