best effort close client

[srperens]

Remove assert that connection closing is successful.

If the jack server is shutdown/restarted while the client connection is up then the closing will fail and you get a thread panic from the assert.

Furthermore the whole client is forever "bricked" since the assert will happen while holding the CREATE_OR_DESTROY_CLIENT_MUTEX.lock(). The mutex will get a PoisonError which prevents a new client connection to be created again. The mutex is a lazy_static "singleton" so it prevents ANY client to be created for the duration of the application.