Issue title:  EMQTT cluster with k8s
 Issue body: #### Environment

- OS: centos7
- Erlang/OTP: 
- EMQ: v3.0.1

#### Description
I used k8s to set up the MQTT cluster, and when my pod was down, all my client connections were down as well. Is there any way to keep the connection up


 Comments: 
 Comment 0: In addition to the above environmental information
The k8s version is v1.13.4
 Comment 1: only one pod down, but bring down all connections to the service? from what you mentioned, it isn't enough clear to me.  Are you using LB? 
 Comment 2: The MQTT protocol is based on TCP long connections, so it is normal for the client to disconnect when the pod is closed.
 Comment 3: Yeah, if the client connected to one pod via service/LB, that client connection got disconnected when pod done, that is understandable. But it seems to me that ysyh15 said one pod done, all client connections disconnected. that is very weird. 
 Comment 4: That is unclear, and would be extremely unusual. EMQX behaves relatively reasonably when losing members from its cluster.
 Comment 5: thx a lot ,
{'source': 'GithubIssues', 'repo_name_with_owner': 'emqx/emqx', 'issue_id': 425821229, 'repo_stars': 7224.0}