Token payload returned by onTokenChange is different in getRole of AdminGuard

### Issue type

**I'm submitting a ...**

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**
Whenever i logout from my app and then login with a different user in the same browser session, the token payload returned inside the login component differs from the token payload returned in getRole() of the admin guard. In particular, the token in the getRole() calls still holds the value of the previous user token, making the process of implementation of the ACL system impossible.

**Expected behavior:**
The token payload is the same in both login component and getRole() call.

**Steps to reproduce:**
Logout from the app, then login with a different user.

**Related code:**

```
// login.component.ts
this.authService.authenticate(this.strategy, this.user).pipe(
takeUntil(this.unsubscribe$)).subscribe((result: NbAuthResult) => {
this.submitted = false;
if (result.isSuccess()) {
this.messages = result.getMessages();
this.authService.onTokenChange().pipe(
takeUntil(this.unsubscribe$))
.subscribe((token: NbAuthJWTToken) => {
if (token.isValid()) {
let payload: any;
payload = token.getPayload(); // here i get the correct token
this.store.dispatch(AuthActions.loginSuccess());
this.initUserService.initCurrentUser().subscribe();
}
});

// role.provider.ts
getRole(): Observable {
return this.authService.onTokenChange()
.pipe(
map((token: NbAuthJWTToken) => {
const payload = token.getPayload(); // here i get the old value of the token payload
return !!( payload && payload ['role']) ? this.getLowerCaseRoles(payload ['role']) : 'guest';
}),
);
}

```

### Other information:

**npm, node, OS, Browser**
```
OS: Windows (10)
Browser: Chrome/Brave
npm: 6.10.1
node: 12.9.1
```

**Angular, Nebular**
```
Angular: 10.2.3
Nebular: 6.2.1

```