Define states and use them to properly handle multiple calls to
`start()`, `stop()`, and `exit()`. (Multiple calls to `exit()` can
happen if there is an uncaught exception or signal during shutdown.)
This should also make it easier to add support for cleanly restarting
the server after a shutdown (for tests or via an `/admin` page).
Rewrite the `callAll` and `aCallAll` functions to support all
reasonable hook behaviors and to report errors for unreasonable
behaviors (e.g., calling the callback twice).
Now a hook function like the following works as expected when invoked
by `aCallAll`:
```
exports.myHookFn = (hookName, context, cb) => {
cb('some value');
return;
};
```
This makes it easier to see the test results, and it hides some
scary-looking but intentional error messages.
This code will likely have to be updated if/when we change the logging
library (see issue #1922).
Richard Hansen