zygote should be able to cleanly restart

[Roguelazer]

Right now, if you want to change the version of zygote itself (as opposed to the app it's hosting), you need to completely stop and start it. This is suboptimal (since it's very slow). It would be preferable of Zygote could use a Unix Domain socket to pass the listening file descriptor over from the old master to the new master for 0-downtime restarts of the master. This might be easiest to implement like haproxy's quick reloads (where you start the new process with an argument of the pid of the old process, and the two communicate behind the scenes to transition versions).