disallow invalid port-removal

do not allow port-removal if the port would be re-added immediately
after that again because the main-delivery actually needs it.

As a side effect this prevents this crash:

 * create a stereo-track, then remove one output
  -> unhandled exception "AudioEngine::PortRegistrationFailure&"

 The problem:
 - the port is removed from the RCU ports list,
   but Port::drop() (which calls jack_port_unregister) is only called
   from the Port's destructor at some later time.
   (because a reference to the port still exists elsewhere)
 - the jack-port is not yet removed.
 - meanwhile Delivery::configure_io comes along and notices that
   there are more audio-buffers than ports and tries to re-register the port.
 - but the port still exists in jack, so it fails and throws an exception
   ...which is not handled.
on session-load: skip output-change-handler until IOs are restored

fixes loading a session with tracks that have more input-ports
than output-ports.

the problem:

input -ports are added first, when added, IO::ensure_port() emits
IOChange::ConfigurationChanged which in turn triggers the route to
allocate corresponding output ports.

Due to this callback the output-ports were created before the actual
output-port-configuration and plugin-configuration from the
session-state was parsed and set.