Make Redis utility methods public

Summary: See https://github.com/nicolasff/phpredis#_prefix.
Closes https://github.com/facebook/hhvm/pull/4489

Reviewed By: @sgolemon

Differential Revision: D1748381

Signature: t1:1748381:1419272599:ab2b00e5d769149f69767c5f4189f32eb31dab20