main: start ruby server after opening network listeners

Fixes #1850 (closed)

Before this change we did this during the main executable startup phase:

• start "ruby server" which asynchronously spawns multiple ruby processes
• open listening sockets

In the case where we failed to open a listening socket, this would exit in the middle of starting ruby processes. That should not be a problem but because of a macOS Go bug we don't fully understand yet, sometimes this is a problem. #1850 (closed) .

The change in this MR makes the scenario where we happened to trigger this bug impossible. We now wait with starting the ruby processes until after we've opened all listening sockets. This is a workaround, not a fix.