Exit now closes the connection rather than exiting the process.
This allows things to shutdown gracefully, and removes special
cases. It also allows the tests to call CloseEditor instead of
just Shutdown, which prevents goroutine leaks.
Change-Id: I26121ba5d393ef74ce0e912611c8b3817e3691ea
Reviewed-on: https://go-review.googlesource.com/c/tools/+/231798
Reviewed-by: Robert Findley <rfindley@google.com>