Bluetooth: Host: Defer `conn.recycled()` to the syswq

We don't own the thread that calls the final `bt_conn_unref()`. The
users might not be aware that they are re-using that poor thread's
stack.

Putting it on the system workqueue gives a predictable stack size and
thread priority for the invocation of the recycled handlers.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>