session.setup must be guarded to avoid double calling

[betocantu93]

If you call setup twice, multiple event listeners are added: authenticationSucceeded invalidationSucceeded

This could happen easily if you place the setup call on any beforeModel, say application route beforeModel, without manual guarding. Any transition that happens in between while executing the beforeModel aborts the transition and triggers a new one, which causes the beforeModel to be triggered again, thus calling setup again. This prob should be guarded from within the lib.

As a work around we extended the service with:

async safeSetup() {
    if (!this._setupIsCalled) {
      return this.setup();
    }
  }

[BobrImperator]

Thanks for the PR 👍
It seems like one alternative could be to use an instance-initializer instead (thinking out loud).

Could you add a test asserting that _setupHandlers can be called only once?

We might want to make sure that the setupHandlers runs only once while restore continues to be called.

Suggested change

	if(!this._setupIsCalled) {
	this._setupIsCalled = true;
	this._setupHandlers();
	return this.session.restore().catch(() => {
	// If it raises an error then it means that restore didn't find any restorable state.
	});
	}
	if (!this._setupIsCalled) {
	this._setupIsCalled = true;
	this._setupHandlers();
	}
	return this.session.restore().catch(() => {
	// If it raises an error then it means that restore didn't find any restorable state.
	});