Not registered Zabbix proxies may cause high traffic/overload Zabbix server network interface

Steps to reproduce:

1. Install several Zabbix proxies (use active mode)
2. Enable PSK based encryption for the proxies
3. Don't add proxies in the frontend for now
4. Wait for an hour
5. Check incoming traffic on Zabbix server

Result:
Each proxy generates ~11 Mbps of traffic. I.e. 10 proxies will push ~110 Mbps. Imagine if you have 1000 of proxies... These not registered proxies may overload Zabbix server network interface easily.

Steps to reproduce:

1. Add these new proxies in the frontend (Administration --> Proxies)
2. Wait for a moment and check incoming traffic graph again. You will see that each proxy will use ~10Kbps now (proxy has no monitored hosts).

Expected:
Not registered proxy should generate less traffic or send configuration request not so frequently.

Notes:

• I didn't test passive proxies
• I didn't test proxies without encryption