Disable heartbeat timer when interval is 0
Disable heartbeat timer when interval is 0

This fixes two bugs:
* when the broker sends a heartbeat interval of 0 it means the client
  shouldn't send any heartbeat frames
* tokio's timer panics if the duration between each interval is 0

swizard0/lapin,Geal/lapin,sozu-proxy/lapin,swizard0/lapin,Geal/lapin