Heads up: Switching from `glide` to `dep`

I just merged https://github.com/cockroachdb/cockroach/pull/16275 which switched from using glide to dep to construct and manage vendor. Some of the motivations, concerns, etc are documented in the PR text and comments.

Day-to-day development shouldn’t notice the change, but those changing our dependencies might want to look over the dep docs. Let me know if you have any questions / see any issues!