avoid race condition when deleting/starting sessions...
avoid race condition when deleting/starting sessions
javascript doesn't guarantee the order of AJAX requests,
so we give `Session.delete` and `Kernel.kill` a callback signature.
Changing the kernel type calls `Notebook.start_kernel`,
which terminates the previous session, if defined,
before starting the new one.
A flag is stored, to prevent multiple simultaneous attempts to start sessions, raising a SessionAlreadyStarting Error,
preventing the spec_changed event from firing.
