Remote controller should clean up after itself

[jedevc]

Raised in #1626 (comment), we need to make sure that the buildx remote controller tidily cleans up after itself, instead of leaking resources forever.

I think we need to work out exactly what the lifetime semantics of the debugging session should be, but generally, I think we should follow:

• Processes are terminated up 1. when they exit, or 2. when their parent session terminates
• A session should terminate 1. when it is explicitly terminated by the user, 2. when its parent controller is terminated (for local, when the calling process ends, for remote, when the server terminates), or 3. after a deadline during which no user interaction has occurred
  • The deadline is useful, so that a user can run a build, watch it fail, and then connect to the session to investigate and invoke, without needing to re-trigger the build.
• The remote server should terminate itself when 1. the user requests it to using kill, or 2. it has no more sessions (so that after the deadlines of all the sessions have expired, we can cleanly exit).

cc @ktock