serve_all(
servers,opts) ->ok,err
Serve one or more server instances in a single LEV event loop.
Enters the LEV event loop once for all given server instances: registers the loop-global signal handlers, binds and attaches every listener, and starts a shared housekeeping coroutine. SIGTERM sets the shutdown flag on every server; accept loops drain within their accept timeout and a 5s timer stops the loop for stragglers.
opts.on_start is called inside the loop after all listeners are bound —
the place for applications to spawn their own periodic coroutines.