Version config variables

[peterkeen]

Config vars should be versioned just like code releases. This probably means the env dir gets timestamped directories just like the release dir.

• Timestamped env dirs with unchanged variables copied forward
• A new app-level directory named versions
• A "version" consists of a sequentially numbered directory in the versions directory, which then contains two symlinks. app points at the timestamped code release in releases, env points at the timestamped env in env. It also contains one real directory named procs, which is where pidfiles live (moved from the code release directory)
• The current symlink points at the current version. The previous symlink can probably go away.
• Add a set of version commands