* Improve emqx_hooks and credentials
1. Modify the return modes of emqx hooks.
Change the return value of hook functions to:
- ok: stop the hook chain and return ok
- {error, Reason}: stop the hook chain and return error
- continue: continue the hook chain
And the return value of emqx_hooks:run/2 is changed to:
- ok
- {error, Reason}
And the return value of emqx_hooks:run/3:
- {ok, Acc}
- {error, Reason, Acc}
2. Treat password as a member of credentials.
Password should be wrapped in the `credentials` data-structure, as the
username/password pair together consists of an authentication method.
There can be some methods using some other credential data (e.g.
a JWT token), and these credential data should also be wrapped in the
the `credentials` data-structure.
An event `client.authenticate` is triggered when an user logs in:
```erlang
emqx_hooks:run('client.authenticate', [], Credentials)
```
A `default callback` that deny/allow any user (according to the
`allow_anonymous` config) should be appended to the end of the
callback chain.
The `credentails` is passed through all of the callbacks, and
can be changed over in this process.
* Refactor emqx hooks return mode
* Remove password from PState
terry-xiaoyu