Upgrade pulsar client from `0.8.3` to `0.8.4` (see [pulsar#61](https://github.com/emqx/pulsar-client-erl/pull/61).

Piror this fix, if the producer client experiences a 'socket error' (but not a normal 'socket close'), it may continue sending data to a clsoed socket without any error handling.
From EMQX dashboard, user may observe that the 'totoal' counter keep increasing, but not the other counters: 'success', 'failed' and 'dropped'.